countdowns, messages, scale/scroll/edit current slide
Use global serverOffset.
Fixed#1645 (wrong time of projector clock).
Show countdown description on projector slide.
Added custom slide projector element class.
Added welcome slide as custom slide.
Added user slide projector element class.
Added clock, countdown ans message projector elements.
Renamed SignalConnectMetaClass classmethod get_all_objects to get_all (private API).
Added migrations to core app.
Fixed and wrote tests.
Updated CHANGELOG.