Switch to a custom event system, where SDL pumps into

To do this we needed to take ThreadVariant out of love.thread, and move it to common.
This also changes event names, instead of 1/2-letter abbreviations, full names!

src/modules/thread/wrap_Thread.h

@@ -27,7 +27,6 @@
 
 namespace love
 {
-	extern StringMap<Type, TYPE_MAX_ENUM> types;
 namespace thread
 {
 	Thread *luax_checkthread(lua_State *L, int idx);