WIP: UI Overhaul

Please do not merge as it's a work in progress!

This is a complete UI redesign for Calls, following these mockups as closely as I can.

This MR is here to ask for feedback on the redesign, as I very probably broke many things (I didn't test receiving a call at all for example). Please do not regard the fact that there is a huge unreviewable commit, I will clean that once the design will be ready, I am asking for a behavior review for the moment. :)

I forced usage of the dummy provider for my tests, feel free to drop that during yours.

Known problems:

• No origin is selected by default.
• The origin selector is visible even if there is one one or less origins.
• The info box in the calls window should be closed when the window is hidden.