Second try of my previous pull request, which was partially reverted due to caused crashes and memory leaks
Merge remote-tracking branch 'upstream/master'
This will (hopefully) solve the memory leaks in dynamic ribbons and image button widgets
Issue #1588

Fixed crashes in track info screen
Check texture cache before adding an icon button's disabled texture
Let all SFX commands be handled by the sfx manager thread (otherwise
events could be handled in the wrong order, e.g. a delete from the
main thread before the play in the manager thread, resulting in a crash).
Should fix #1511 and #1604.
Merge branch 'master' of https://github.com/marcoll/stk-code into marcoll-master
Use case-insensitive comparison for animated textures, for some reason
Merge branch 'marcoll-master'
Merge remote-tracking branch 'origin/master'
Removed status check when queueing sfx, to get more consistent
runtime behaviour.
Added computation of the duration of a sound buffer (unused atm).