Bug 414072 - [Viewers] Add generics to Editors

Added generics to the EditingSupport, ICellModifier and corresponding
classes, to allow a
typesafe use of the viewers framework.

Includes also updates in the TableViewer, (Abstract)TreeViewer,
ColumnViewer

Change-Id: I0057e06261d37302984c18c9584bb0d2028ffb5f
Signed-off-by: Hendrik Still <hendrik.still@gammas.de>