msys2: fix build instructions, improve library bundling <3 @Mario

• fix MSYS2 build instructions
• fixes bundling on MSYS2 (libs, gtk2 loaders, engines and themes)
• install the bundle
• let cmake install the downloaded gamepacks itself
• increase resolution of netradiant icon and use it as exe icon
• some more stuff (q3map2 icon etc.)

The install_manifest.txt file must now contain all installed files, including gamepacks, not only on msys2.