Set display-fps as a property

This means the player core will actually be notified of the change,
should it happen during playback.

(These are all artifacts of the property/option separation, which
increasingly stops making any sense.)

Requires mpv git not older than a week.

src/player/PlayerComponent.cpp

@@ -761,7 +761,7 @@ void PlayerComponent::updateVideoSettings()
 
 #ifndef TARGET_RPI
   double display_fps = DisplayComponent::Get().currentRefreshRate();
-  mpv::qt::set_option_variant(m_mpv, "display-fps", display_fps);
+  mpv::qt::set_property_variant(m_mpv, "display-fps", display_fps);
 #endif
 
   setAudioDelay(m_playbackAudioDelay);