Cleanup & refactor

Move Angelscript addons to a new dir + create project for it (static)
Move Angelscript bindings to a new dir (separate script bindings from engine internals)
Small cleanup on the serverbrowser interface + set the serverbrowser tab as the default one (generally that is the tab people want to open when playing)
Created 'Sources/Externals' directory where sdl, zlib and all external projects can be dropped (mainly for windows users having problems where cmake cannot find the libraries)