Mod profiling support

Config settings:
profiling = true/false (gather statistics)
detailed_profiling = true/false (break mod times to callbacks)

Chat commands:
save_mod_profile saves current statistics in debug.txt and shows on console (on default loglevel)
Fix caption of config mods button (simple menu)
Merge remote-tracking branch 'minetest/master'

Conflicts:
	doc/lua_api.txt