Added support for annotating proven methods

This commit also tweaks the normal Java-editor annotation code a bit. I
should really try to unify the annotation behaviour of the two editors
Unified the annotation-rendering code of the Coq and Java editors
Added support for command handler registration to JavaEditorState
Moved the Java editor handlers and jobs into separate files

To keep things consistent, this commit also renames the files containing
the Coq editor handlers and jobs
Added a new base class for all editor handlers

This class only provides a sensible implementation of setEnabled for now
Stopped using the deprecated org.eclipse.ui.popupMenus extension point

This change allows ProveMethodAction to be reimplemented as a normal
IHandler, and so this commit also does that and removes the KAction class
Moved the Java proof initialisation handler into JavaEditorHandlers

No actions remain in Actions.scala, so this commit also renames that file
to JavaEditorState