refactor: various logic improvements

[Samillion]

Changes:

• Change screenshot button default value to only show video
• Cache hover effects instead of calls on every frame
• Move format_time() and build_cache_seek_ranges outside of osc_init()
• Use already cached state.chapter_list instead of re-calling property various times
• Use mp.get_property_bool() instead of redundant string check mp.get_property() == yes/no
• Remove pause_state() function, didn't need to exist in the first place
• Simplify mouse init and pos checks in process_event()
• Reset state.visibility_modes before populating
• Don't retrieve display-fps at startup as it doesn't work.
  • follows upstream mpv