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.discord.in

@@ -0,0 +1,12 @@
+[Desktop Entry]
+Type=Application
+Version=1.0
+Name=OpenRA - {MODNAME}
+GenericName=Real Time Strategy Game
+GenericName[de]=Echtzeit-Strategiespiel
+Icon=openra-{MODID}
+Exec=openra-{MODID} %U
+Terminal=false
+Categories=Game;StrategyGame;
+StartupWMClass=openra-{MODID}-{TAG}
+MimeType=x-scheme-handler/openra-{MODID}-{TAG};x-scheme-handler/discord-{DISCORDAPPID};