Medal/icon system overhaul and MEDALDEF special lump

This is a huge overhaul of the medal (and partly the icon) system that's existed since Skulltag. Here are the notable changes:

• Added a new MEDALDEF special lump that allows modders to define their own medals.
• Modders can award players with players with the new ACS and DECORATE functions: GivePlayerMedal and A_GivePlayerMedal, with an option to silently award the player with the medal if they want to.
• Removed the GIVEFISTINGMEDAL and GIVESPAMMEDAL actor flags that were added earlier in 3.2.
• Returned the enemy icon to Zandronum, with new CVars to enable/disable the ally and enemy icons. By default, enemy icons are disabled.
• Added new ally/enemy icons made by unknownna that look more modern and less intrusive.
• Added a new flag to the GAMEMODE lump: PLAYERSEARNMEDALS, to determine which game modes allow players to earn medals. By default, only non-cooperative game modes have this flag enabled.
• New SCORINFO margin commands: DrawMedals and IfPlayerHasMedals, both of which are used to draw all of the medals the player has earned on the scoreboard. This replaces and removes the +showmedals screen, which is no longer necessary to keep around.

Here's a screenshot of how the medals look on the scoreboard:

And here's another screenshot of how the new ally/enemy icons look: