Major changes 1.12 => 1.15

Clean-ups & updates:
- Spring 84.0 ready: updated many gadgets and widgets to be compatible with the recent spring engine version
- replaced the "old" Survival Mode with the brand new Zombie Survival Mode
- added UI elements for the Survival Mode
- added map-borders (an effect to make maps look like being part of a bigger "world")
- AI controlled ghouls and skeletal mages will jump/teleport in survival mode now
- implemented a LuaAI for skirmish games (CAI - from Zero-K)
- added a parry system for melee untis (paladin, ghoul and skeleton)
- merged and updated the jumping and teleport gadgets to work smoother
- added a mod option to define how long corpses should stay on the battlefield
- added a new experience system: units will gain XPs for all killed enemies within a certain range

Balance:
- melee weapons only do 50% damage against heavy armour (this will make early ghoul rushes more ineffective)
- altered the imperial marine's fire mode
- added advanced walls for the cursed
- many small balance changes were performed (adv. artillery ranges, some costs,...)

Bugs:
- AOE units (like the imperial pyros) won't hurt themselves anymore

Polishing:
- updated the user interface (using the ZeroK-selection menue)
- updated some sounds
- updated some explosion effects (using stuff from Smoth's GundamRTS)
- added some new normal maps

Installer:
- removed all skirmish AIs except the LuaAI (for compatibility reasons)