Add hotkey option to display the media title

This adds the ability to set a hotkey that shows the current media title on the OSD, the same that is shown when media starts playing and the video-title-show option is true.

Fix #7967

Note: If this approach is acceptable, I can squash the commits to not have the unnecessarily late API change, but do it together with the refactor commit.

A different approach to exposing vout_DisplayItemTitle would be to add a new function to the player, vlc_player_DisplayPosition(player, item); and call the resource's DisplayVoutTitle there.