don't crash the asset browser when loading sprites with 1 frame

closes #4125
StyleCop
harden asset browser against invalid user input and allow R8

closes #3980
abbreviate the user support folder as in mod.yaml
added a palette chooser and colorpicker dropdown to the browser
PaulCop clean
updated CHANGELOG
Merge pull request #4151 from Mailaender/assetbrowser-renovation

Asset Browser Renovation
Make corpses always appear when infantry die in ra/cnc.
Increase corpse duration