Better soundfont packaging for fluidsynth

It came to my attention that fluidsynth by default doesn't have any soundfont installed alongside it. This is easy to debug if you launch it directly from console, but if it's used by another application (like testing/openttd w/ testing/openttd-openmsx), it fails almost silently.

Furthermore, the only other soundfont we've had packaged (testing/soundfont-vintage-dreams-waves) is by far the.. weirdest MIDI soundfont I've heard, and it doesn't work any well for reproducing most songs.

This MR currently packages soundfont-timgm, which is a free soundfont that works "well enough" (IMO it sounds on par with Microsoft GS Wavetable Synth). To ensure better compatibility, timgm is aliased as default.sf2 in /usr/share/soundfonts/.

As fluidsynth is virtually useless without a soundfont, I propose that this package should be a dependency for it - but before that can happen, it would need to exist in community instead of testing. What would be the course of action here?