Started separating application and window menu.

This is still a WIP with many broken things.
Main Window: Simplified toolbar code and moved to gear menu style.
Prefs: Added option to control the display of offline contacts.
Main Window: Dropped useless variable.
Main Window: Style adjustment.
Ekiga: Moved preview control to EkigaApplication.

This looks pretty logical to control preview display from the main
application and not from the VideoInputCore directly as it requires the
call window to be displayed before starting preview.
EkigaApplication: Removed menubar from the EkigaApplication definition.