frontend: introduce generics to UICommon (part 2)

Infrastructure part of the introduction of the generics into UICommon

Changes:
- Changed the SearchableListModel to be generic (only uses type <T> but as soon
  as the children will be refactored to be generic than it will change to
  <T extends IVdcQueryable>
- created the generic versions of the following widgets:
  EntityModelLabel, EntityModelTextAreaLabel, EntityModelPasswordBox,
  EntityModelTextArea, ListModelSuggestBox

Change-Id: Ic835d37aba4821d2497652177ae84b1fd37ba61d
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>