Increased maximum number of file transfers to 32.
Add optional Ringtone to uTox. Can be enabled in the settings page.
Merge branch 'Ringer' of https://github.com/bob3434/uTox

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Ringtone fixes.
Ringtone fixes: use looping source.
ringtones should now work with multiple callers.
Only generate the ringer sound once, clean up after.