Add support for multiple soundfonts

[cheap-glitch]

Your environment

• Version used: 0.9.5.245 (latest develop)
• Platform used: JavaScript
• Rendering engine used: SVG
• Browser Name and Version: Firefox
• Operating System and version (desktop or mobile): Arch Linux desktop

Expected Results

Muted strings and metronome clicks should be played correctly when using a guitar-only soundfont.

Observed Results

Guitar-only soundfonts have trouble with muted strings: instead of a "click" they produce a very low sound. The same problem appears when activating the metronome: instead of discrete clicks there is a continuous low note playing.

Steps to Reproduce (for bugs)

Create a tablature with the following soundfont:
soundfont.zip

Possible Solution

Having a way to specify a second soundfont to play the percussion sounds would be awesome. Ideally it'd be a soundfont solely made for that purpose so it could stay lightweight, but I know nothing about the sf2 format so I don't know if this is at all possible.

[Danielku15]

Sounds like a good idea to support loading of multiple soundfonts individually to tailor the output better. Supporting it should not be a big deal but rather a topic for 1.x .

Implementation Notes:
Internally we hold a list of presets that are derived from the soundfont. from this patches we try to find a matching one. Wrapping up the patches into a SoundFont container and extending the patch selection logic should be fairly easy.

[Danielku15]

Multiple SoundFonts can now be loaded.

New APIs:
https://www.alphatab.net/docs/reference/api/loadsoundfont/ (new parameter)
https://www.alphatab.net/docs/reference/api/resetsoundfonts/ (new call)

Guide:
https://www.alphatab.net/docs/guides/multiple-soundfonts