Load fluidsynth entrypoints dynamically (and make them optional)

This removes the static dependency on fluidsynth being present
at buildtime (even headers aren't needed anymore).

Even though midi is a default format for the RPG XP/VX series,
it has fallen more and more out of use, with VX Ace completely
abandoning it from the RTP and making ogg vorbis the de facto
standard. Midi support is kept for legacy reasons, but isn't
encouraged. On top of all this, fluidsynth together with glib
is a heavy dependency that often times won't even be used.
Making it optional at build time is an attempt to unify and
keep build config fragmentation low.

In RGSS3, fluidsynth / midi is not initialized at all by
default, but rather on demand when either a midi track is
played back or Audio.setup_midi is called.

src/filesystem.cpp

@@ -340,10 +340,8 @@ FileSystem::FileSystem(const char *argv0,
 	if (rgssVer >= 2 && !contains(p->extensions[Audio], std::string("ogg")))
 		p->extensions[Audio].push_back("ogg");
 
-#if MIDI
 	p->extensions[Audio].push_back("mid");
 	p->extensions[Audio].push_back("midi");
-#endif
 
 	/* Font extensions */
 	p->extensions[Font].push_back("ttf");