Gravity proc tweaks (#19047)

Tweaked gravity check proc to be more inline with TG's version.
Added signals for gravity check, that allows atoms to perform
anti-gravity.
Fixed roller beds thinking they have no gravity and thus not emitting
the rolling sound.

Fixes #19045

code/modules/mob/living/maneuvers/_maneuver.dm

@@ -13,7 +13,7 @@
 		if(!silent)
 			to_chat(user, SPAN_WARNING("You are buckled down and cannot maneuver!"))
 		return FALSE
-	if(!has_gravity(user))
+	if(!user.has_gravity())
 		if(!silent)
 			to_chat(user, SPAN_WARNING("You cannot maneuver in zero gravity!"))
 		return FALSE