Add lookup table for Hedgewars key IDs instead of generating IDs on the fly
This makes sure the key IDs never change. Previous implementation abused SDL_GetScancodeName to generate the IDs programmatically, which was not reliable and explicitly adviced against in the SDL documentation <https://wiki.libsdl.org/SDL_GetScancodeName#Remarks>.
07f3bf8d98a3
Hedgewars.png had a resolution more than twice as high as required; also fixed some (mostly obsolete) code that also contained a typo that would cause a warning/note in clang
