Brings a lot of features (icons, keyboard shortcuts,
customization, context menu interception etc.)

Other changes:

- Make SfxDispatcher::ExecutePopup take PopupMenuFlags
  arg, to be able to pass NoMouseUpClose, so that the
  menu won't close on mouse-up under anything != gtk3.
- Make .uno:OpenFromWriter and .uno:OpenFromCalc async
  slots, to avoid crash under gtk3 when recycling the
  current frame.
- Don't show the menu in customization dialog, if
  experimental mode disabled.

Change-Id: Ieea2f73f6b48fbaf4bdfc4acf1a33d25422357cb
Reviewed-on: https://gerrit.libreoffice.org/33101Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Maxim Monastirsky <momonasmon@gmail.com>