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.
Changed api for main menu entries.
Enhanced http error pages using a classed based views.
Moved dashboard and select widgets view from projector to core app.
Also some small clean ups.
It is now possible to define a default state and to submit extra stylefiles and javascript files when defining a widget in an app. This is done by a new metaclass in openslides.utils.dispatch. Also fixed some other tests.
