Added support for LOOPSTART and LOOPLENGTH tags

[ThePotatoGuy]

#1670 #1657

Okay @williamjcm , are you satisfied now? (jk, no animosity here, in case it wasn't obvious)

This changes the tags introduced in #1670 to be masloopstart and masloopend, and adds support for the RPGMaker tags LOOPSTART LOOPLENGTH (which are samples instead of the superior seconds).

LOOPSTART and LOOPLENGTH tags only work for OGG/VORBIS files

Looping instructions:

MAS-style:

1. Open audio file in audacity
2. Export as OGG / OPUS and edit metadata
3. Add a tag for masloopstart and specify the timestamp (in seconds) where to start the loop.
4. Add a tag for masloopend and specify the timestamp (in seconds) where to end the loop.
5. Put audio file in custom_bgm/ folder.

RPGMaker-style:

1. Open audio file in audacity
2. Export as OGG and edit metadata
3. Add a tag for loopstart and specify the number of samples where to start the loop.
4. Add a tag for looplength and specify the number of samples for how long the loop should last.
5. Put audio file in custom_bgm/ folder.

NOTE:

• all tags are case-insensitive
• MAS-style tags are preferred over RPGMaker-style tags