More merges from Andreas Zenner:
External 3D model improvements:
* Models/Instruments/Canopy/MiG-15bis-Canopy.ac: rewrite.
* Models/Instruments/Canopy/MiG-15bis-Canopy.xml: simplify animations.
* Models/Instruments/Canopy/uv_canopy_[01].png: new.
* Models/MiG-15bis.ac: rewrite to remove seams and allow different
textures on either side of ailerons, trimmer, etc.
Livery support:
* Models/Liveries/CSSR_aerobatic/*: new livery from Andreas Zenner.
* Models/Liveries/USSR_211/*: move the old (default) livery here;
update textures to match the new seamless model.
* MiG-15bis-set.xml: add menu item to change liveries.
* Models/MiG-15bis.xml: load livery support when a remote or AI player
appears and flies a MiG-15.
* Nasal/MiG-15bis.nas: initialize livery support on startup.
Additional bug fixes:
* Models/Instruments/Turnometer/MiG-15bis-Turnometer.xml: indicate
rate of turn (yaw), not rate of roll.
* Nasal/bombable.nas: fix a typo.