Replaced VexWidget#dispose with a DisposeListener

The dispose method is not called, when the VexEditor is closed, so
instances of VexWidget remain on the heap.
According to the javadocs of Widget#dispose we now use a listener
instead. 

Change-Id: I41a9fabb6d5397123dc297bfcc10313e78deb4fb
Signed-off-by: Carsten Hiesserich <carsten.hie@gmail.com>
2 files changed