added in-game SHP viewer with frame slider
added the game asset viewer to d2k mod
remove duplicate ra-desert bridge tiles
added .mix file support for game asset browser
added button to extract the selected SHP and convert it to PNG
add a dump everything to PNG options for @xanax
fix Makefile

OpenRA.Mods.RA.dll now depends on the Utility.exe
added MinimumThumbSize to ScrollPanelWidget

closes #2865
game asset browser UI polishing

- display file extension
- show total frame count
- dark background for preview
wire up the import from PNG to SHP button

store everything in user directory to not junk up the game
and for file permissions on Mac/Linux when installed
adapt asset browser for R8 files