Build broken - xtend-maven-plugin must run after *.mwe2!

When I try this out (as of https://github.com/svenefftinge/maven-xtext-example/commit/e48ec465d93811a407825659316d421146b283c9), as in CLI git clone the repo, without any Eclipse IDE with some Xtend Builder kicking in, like e.g. a CI server would, then it doesn't actually work... I'm getting the error shown below.

This is because the xtend-maven-plugin is running BEFORE the exec-maven-plugin ran the *.mwe2 - which cannot work.

This PR fixes that, by declaring the xtend-maven-plugin only in pluginManagement instead of plugin in the parent pom.xml, and then listing it (only with groupId & artifactId; executions and configuration are inherited) in the core/ui/test pom.xml AFTER the exec-maven-plugin (in the core) or BEFORE the tycho-surefire-plugin (in .test).

If you / anyone else knows of a better way than this to control the execution order of plugins, avoiding redeclaration of xtend-maven-plugin in 4 places, that would obviously be even better - but this works.


```[INFO] ------------------------------------------------------------------------
[INFO] Building My Hero Language Core 1.0.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO] 
[INFO] --- maven-clean-plugin:2.4.1:clean (default-clean) @ my.mavenized.herolanguage ---
[INFO] Deleting /home/vorburger/dev/3rd-party/maven-xtext-example/my.mavenized.herolanguage/target
[INFO] 
[INFO] --- tycho-packaging-plugin:0.15.0:build-qualifier (default-build-qualifier) @ my.mavenized.herolanguage ---
[INFO] 
[INFO] --- tycho-packaging-plugin:0.15.0:validate-id (default-validate-id) @ my.mavenized.herolanguage ---
[INFO] 
[INFO] --- tycho-packaging-plugin:0.15.0:validate-version (default-validate-version) @ my.mavenized.herolanguage ---
[INFO] 
[INFO] --- xtend-maven-plugin:2.5.0-SNAPSHOT:compile (default) @ my.mavenized.herolanguage ---
[ERROR] 
ERROR: 	HeroLanguageValidator.xtend - /home/vorburger/dev/3rd-party/maven-xtext-example/my.mavenized.herolanguage/src/my/mavenized/validation/HeroLanguageValidator.xtend
12: AbstractHeroLanguageValidator cannot be resolved to a type.
[ERROR] 
ERROR: 	HeroLanguageValidator.xtend - /home/vorburger/dev/3rd-party/maven-xtext-example/my.mavenized.herolanguage/src/my/mavenized/validation/HeroLanguageValidator.xtend
12: Superclass must be a class
[ERROR] 
ERROR: 	HeroLanguageJvmModelInferrer.xtend - /home/vorburger/dev/3rd-party/maven-xtext-example/my.mavenized.herolanguage/src/my/mavenized/jvmmodel/HeroLanguageJvmModelInferrer.xtend
4: my.mavenized.heroLanguage.Heros cannot be resolved to a type.
[ERROR] 
ERROR: 	HeroLanguageJvmModelInferrer.xtend - /home/vorburger/dev/3rd-party/maven-xtext-example/my.mavenized.herolanguage/src/my/mavenized/jvmmodel/HeroLanguageJvmModelInferrer.xtend
47: Heros cannot be resolved to a type.
[ERROR] 
ERROR: 	HeroLanguageJvmModelInferrer.xtend - /home/vorburger/dev/3rd-party/maven-xtext-example/my.mavenized.herolanguage/src/my/mavenized/jvmmodel/HeroLanguageJvmModelInferrer.xtend
49: The method name is undefined for the type HeroLanguageJvmModelInferrer
[ERROR] 
ERROR: 	HeroLanguageJvmModelInferrer.xtend - /home/vorburger/dev/3rd-party/maven-xtext-example/my.mavenized.herolanguage/src/my/mavenized/jvmmodel/HeroLanguageJvmModelInferrer.xtend
51: The method name is undefined for the type HeroLanguageJvmModelInferrer
[ERROR] 
ERROR: 	HeroLanguageJvmModelInferrer.xtend - /home/vorburger/dev/3rd-party/maven-xtext-example/my.mavenized.herolanguage/src/my/mavenized/jvmmodel/HeroLanguageJvmModelInferrer.xtend
52: The method someCode is undefined for the type HeroLanguageJvmModelInferrer
[ERROR] 
ERROR: 	HeroLanguageJvmModelInferrer.xtend - /home/vorburger/dev/3rd-party/maven-xtext-example/my.mavenized.herolanguage/src/my/mavenized/jvmmodel/HeroLanguageJvmModelInferrer.xtend
52: Ambiguous feature call.
The extension methods
	setBody(JvmExecutable, StringConcatenationClient) in JvmTypesBuilder,
	setBody(JvmExecutable, Procedure1<ITreeAppendable>) in JvmTypesBuilder and
	setBody(JvmExecutable, XExpression) in JvmTypesBuilder
all match.
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO] 
[INFO] parent ............................................ SUCCESS [3.667s]
[INFO] My Hero Language Core ............................. FAILURE [8.013s]
[INFO] My Hero Language UI ............................... SKIPPED
[INFO] My Hero Language Tests ............................ SKIPPED
[INFO] My Hero Language Feature .......................... SKIPPED
[INFO] My Hero Language Update Site ...................... SKIPPED
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 40.834s
[INFO] Finished at: Sat Nov 23 00:27:53 CET 2013
[INFO] Final Memory: 113M/605M
[INFO] ------------------------------------------------------------------------```

This is because