Optimized minetest.get_connected_players()

Instead of collecting all objects within a huge radius (which could be
a big value), just register each player that connects and give back the
current hold list.
Remove mapgen_air alias (#935)
FIx wrong error message on invalid use of the formspec element image_button
Add Czech language
Translated using Weblate (German)
Translated using Weblate (French)
Translated using Weblate (Dutch)
Translated using Weblate (Chinese (China))
Translated using Weblate (Portuguese (Brazil))
Translated using Weblate (Dutch)
Translated using Weblate (Portuguese (Brazil))
Translated using Weblate (Spanish)
Merge remote-tracking branch 'upstream/master' into weather

Conflicts:
	src/content_abm.cpp