Don't add gwt-dev twice in gwt:run-codeserver classpath

gwt-dev (and deps) is already added via gwt-codeserver.
Update archetype

 - remove gwt:i18n
 - use com.google.gwt:gwt BOM
 - include gwt-dev and gwt-codeserver as project dependencies
 - update JUnit
 - remove unneeded javax.validation
Remove all scripts to install GWT SDK into Maven repo

GWT comes with its own scripts if you compile it yourself, and now
publishes Maven artifacts (including snapshots)
Why the hell did gwt:test mode default to manual?!
Remove c.g.g.junit.JUnit from the archetype-generated gwt.xml
Revert "Remove default extraJvmArgs value: modern JVMs have better memory defaults"

It turns out JVMs aren't that smart and the GWT Compiler will happily
eat all your memory making your desktop unusable for a few minutes.

This reverts commit 0ac3f62758d8d356b3800668e7831120de8e7af0.
Attempt at fixing artifact transitive resolution.

Once again just copied some code from another plugin as I couldn't find
any documentation (and javadoc seems to be forbidden on Maven APIs),
hoping for the best (tests still don't pass, yay!)