Add some integ test coverage for incremental behaviour of ScalaLang
refactor ScalaCompile, PlatformScalaCompile tasks

- put common logic into AbstractScalaCompile
- create Compiler for PlatformScalaCompile in ScalaToolProvider
- no need for ScalaToolChain#getScalaClasspath + ScalaToolChain#getZincClasspath anymore
- use hardcoded Scala version in 2.10.4
+review REVIEW-5273
Extract ScalaToolProvider in seperate class

- some more cleanup
- give decent error for unsupported CompileSpecs in ScalaToolProvider

+review REVIEW-5273
provide meaningful error message when triggering PlayToolProvider#newCompiler with unknown CompileSpec type

+review REVIEW-5258