Major track length-related updates.
- Parse and store the end time for every beatmap (i.e. end time of last hit object).
- Added a 'length' sorting tab.
- Added 'length' search condition.
- Removed 'getTrackLength()' and 'getTrackLengthString()' methods, as they are no longer needed.
- Added a loader spritesheet animation to render during MP3 conversions (in place of track length rendering upon completion).

Other changes:
- Added a yellow progress circle during lead-in time.
- Fixed sorting tab positioning.
- Slightly increased button animation speed in "Main Menu Exit" state.

Signed-off-by: Jeffrey Han <itdelatrisu@gmail.com>