Update WebAppCreator templates

- use Java 7 by default
- use gwt-maven-plugin with the same version as GWT. We'll just
  hope that it's already released when the user runs WebAppCreator.
- remove extraneous Maven dependencies and plugin configurations
- there's a platform-neutral gwt-dev.jar for quite some years now
- add -war in Ant and Eclipse devmode tests' gwt.args, same as prod mode
- remove all Xmx, modern JVMs have sane defaults
- remove superDevMode target in Ant, and add gwt-codeserver.jar to devmode
- DTDs are now published at gwtproject.org
- make all pathelements relative to ${gwt.sdk} in Ant
- use BOM in dependencyManagement in Maven

Change-Id: Ibfdbe470a4da35ed53c1340aed403d9847ddcf35
Update samples to use 2.7.0-SNAPSHOT of GWT and gwt-maven-plugin

Change-Id: I81806b92bd33c8b2aaeb6e41edcfd0aaa5cd6aa4
Cleanup samples' POMs and update some dependencies

Change-Id: Ifebac29bf3337e3e68dfbac00d74caaea088fa52