Video playback with mpv

Add video playback with mpv

• Add the mpv widget
• Enable the widget only for video able documents
• Control the widget with a small set of buttons
• Seek mpv with slider that indicates the current position in the video
• Use std::chrono in the MpvControls to be sure to have the correct time unit...

Not mpv related changes

• Don't del and add docks if they are already present, just swap the contained widget => don't reset the position of the dock at sub-document updates

What it does not for now :

• Get rid of controls on MPV view when the dock goes fullscreen