QAFE-980 [READY] [S] As QAFE developer I want to execute an event with
"close-panel" built-in on the client-side when a button is clicked so
that I can execute "close-panel" without server-side round-trip.
- Added ClosePanelHandler for handing client-side events
- Added ClosePanelAssembler for sending server-side components to
client-side
- Moved helper-functions used in both ShowPanelHandler and
ClosePanelHandler to BuiltInHandlerHelper.