Bug 412273 - [Viewers] Add generics to the ComboViewer

Generics are added to the ComboViewer to provide a type safty use of
this viewer

This change provides also the base implementation for the conversion of
other viewers

Change-Id: Idff3492a4f8eb54762b3e0ce54d635d5cbfff282
Signed-off-by: Hendrik Still <hendrik.still@gammas.de>