backend: Remove unused OSD popup
backend: Remove unused function from header
backend: Allow forcing the popup to stay on
main: Don't hide popups when menu is popped out
backend: Hide popup only once playback has started
backend: Fix incorrect time label when starting playback

We weren't changing the label when the length of the film was known,
so we had the hour field missing on one side, and 1:23:00 changing on
the other, until we reached the first second of playback.