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

src/UI/DocumentViews/MpvContainer.cc

@@ -60,6 +60,7 @@ void
 MpvContainer::closeMpv() noexcept
 {
     if (mpv) {
+        qDebug() << "Closing the MPV context";
         registerMpvTimeCallback(nullptr);
         registerMpvDurationCallback(nullptr);
         mpv_handle *tmp_mpv = mpv;