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
Merge branch 'master' of https://github.com/marcoll/stk-code into marcoll-master
Merge branch 'marcoll-master'
Merge remote-tracking branch 'origin/master'