Bug 413973 - [Viewers] Add generics to the TreeViewer

Currently this is a public draft to discuss this changes.
See discussion on the platform-ui-dev mailinglist:
http://dev.eclipse.org/mhonarc/lists/platform-ui-dev/msg05452.html

Change-Id: I21bb51ccca4625455e56d59766cc687f306bef97
Signed-off-by: Hendrik Still <hendrik.still@gammas.de>