Switch opl2 to submodule

[tresf]

Switch the "OpulenZ" (opl2) instrument plugin to pull directly from https://github.com/adplug/adplug.

There are some commits that have been added to the codebase that never made it upstream however upstream has been maintained as well, so some of them are redundant to apply.

One of my concerns is C++11 compatibility, specifically the use of the __inline combined with Clang, which caused problems in the past. Edit:, upstream removed __inline, may be OK.

Here's a full history of our (downstream) changes to the fmopl.c library. These will help resolve issues that occur as a result of this merge.
https://github.com/LMMS/lmms/commits/stable-1.2/plugins/opl2/fmopl.c

Tasks

• Switch opl2 to use subdirectory
• Rename plugin directory to "OpulenZ" to match convention, write upgrade task.
• Handle edge-case compilation issues

@softrabbit FYI.

[tresf]

There doesn't appear to be any issues with the build, even on Apple. Merging.