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

@@ -179,6 +179,9 @@ MpvContainer::onMpvEvent() noexcept
 void
 MpvContainer::loadFile(const QString &filename) noexcept
 {
+    if (filename.isEmpty())
+        return;
+
     const QByteArray c_filename = filename.toUtf8();
     const char *args[]          = { "loadfile", c_filename.data(), nullptr };
     printMpvError(mpv_command_async(mpv, 0, args));