Adds Support for refreshable trees

Besides the RefreshComposte a tree can now be enhanced with a
RefreshHandler. This is done by calling Widgets.onTree( tree
).setRefreshHandler( ... ).

A RefreshHandler basically works the same way a RefreshComposite works
except it has no own bounds.

Change-Id: Id1c85878f75104cae01d06242a7d7aa47f618892
Merge "Adds Support for refreshable trees"