Overhaul packaging scripts and helpers:

- Shared helpers extracted to functions.sh for use by upstream packaging,
  Mod SDK, and downstream packaging (via the Makefile targets).
- Assembly management separated from data and combined between engine
  and mods to prepare for pending .NET core requirements.
- Streamline Makefile targets.
- Clean up a lot of old technical debt.

packaging/linux/openra.desktop.in

@@ -4,9 +4,9 @@ Version=1.0
 Name=OpenRA - {MODNAME}
 GenericName=Real Time Strategy Game
 GenericName[de]=Echtzeit-Strategiespiel
-Comment=Reimagining of early Westwood Games
 Icon=openra-{MODID}
 Exec=openra-{MODID} %U
 Terminal=false
 Categories=Game;StrategyGame;
-MimeType=x-scheme-handler/openra-{MODID}-{TAG};x-scheme-handler/discord-{DISCORDAPPID};
+StartupWMClass=openra-{MODID}-{TAG}
+MimeType=x-scheme-handler/openra-{MODID}-{TAG};