webadmin: Modified EntityModelCellTable for input columns

Added support in the event handling mechanism of EntityModelCellTable
for CellTable columns that prefer to handle their own browser
event. Such columns' cells should implement EventHandlingCell, and
then implement the method handlesEvent() to decide whether they'd like
to handle a given event themselves.

For now, made ListModelListBoxCell and CheckBoxColumn handle their own
click events.

Also, changed EntityModelCellTable to only treat the selection column
checkbox as a selection switch, and let other checkboxes do their own
thing.

Change-Id: If46c72cea581d331fea030ea9f2ae2f42129f44f
Signed-off-by: Lior Vernia <lvernia@redhat.com>