Fix manually adjusting playstate + minor improvements

[PickleUpADoodle]

This pull request fixes a bug where the rich presence would not be updated if the playstate had manually been altered by the user. As a result, the elapsed time could grow indefinitely if the user kept restarting a song, or the time left would stay 0 if the original time left was expired.

Also includes some code quality improvements:

• Remove global variables
• Change variable and method names to use snake case
• Remove unnecessary "compareTab" in App.py
• Change "use_time_left" to bool instead of string
• Use time.time() once instead of calling it whenever we need the current time

[manucabral]

Great implementation!