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.
* agenda_item.get_absolute_url('projector') returns the activate-url of the related slide.
* agenda_item.is_activate() returns True, if the related item is active
* Fixed set_active_slide to accept kwargs
* Reset countdown when saving a new duration time
* Update countdown overlay when begin_speak and end_speak is called
* Fixed blinking countdown
Fixes: #1078, #1076, #1075
