Make portsmf a git submodule

[rubiefawn]

This makes portsmf, which is used in the MIDI import plugin, a git submodule.

[rubiefawn]

Marking this as a draft while I test MIDI import for regressions. I have no idea what version of portsmf was previously being used vs whatever the current version is, so it could be completely broken for all I know.

[tresf]

I have no idea what version of portsmf was previously being used vs whatever the current version is, so it could be completely broken for all I know.

Here's the diff.
https://gist.github.com/tresf/8427469c7f2ea670bb0e56f36cf8f349

Here's the commit history in LMMS:

• After rename: https://github.com/LMMS/lmms/commits/9b04e29c4e416449abe7a6a1b03a2e934a9f3633/plugins/MidiImport/portsmf
• Before rename: https://github.com/LMMS/lmms/commits/0055fe13b1f672a5d17db57c907bc764d744b676/plugins/midi_import/portsmf/allegro.cpp

The diff is probably enough to hand-patch and provide it upstream, however in doing so, it would probably make sense to do it for the entire project (not just the files we use from it).

When working on #3963, we had to do this for each several submodules. Fortunately for this one, most of the changes appear to be rather trivial.

[rubiefawn]

It looks like my initial PR for upstream that fixes the compiler warnings got merged, so that's nice! I'll update the submodule.

[rubiefawn]

I'm going to temporarily point the submodule at my fork while I fix all this, and then make a PR against upstream once it's compiling fine in here