Reverse engineered many of the multiplayer events

Some corrections to the way events are handled and identified.

include/constants/battle_frontier.h

@@ -10,6 +10,8 @@
 #define FRONTIER_FACILITY_PIKE 5
 #define FRONTIER_FACILITY_PYRAMID 6
 #define NUM_FRONTIER_FACILITIES 7
+// The multiplayer battle colosseum rooms re-use VAR_FRONTIER_FACILITY.
+#define FRONTIER_FACILITY_DOUBLE_COLOSSEUM 9
 
 #define TENT_VERDANTURF 2
 #define TENT_FALLARBOR 3