macosx: Make the video view dismissible while retaining playback in background in main library window

This PR lets the user use the backward navigation button to dismiss the video view in the main library window. The video still continues playing in the background.

The video view can be brought back by playing a new video, changing the playback state to be playing, and clicking the thumbnail in the main window control bar.

Screen_Recording_2022-09-04_at_18.43.33

Screen_Recording_2022-09-04_at_19.25.01

Signed-off-by: Claudio Cambra claudio.cambra@gmail.com