Draw Grid Mode: fixed a problem where "continuous drawing" would redraw all previously drawn shapes each time a drawing was finished. Fixes #912

UDBScript: fixed an issue where the Thing's class getSector method did not return null when the thing was not in a sector. Fixes #897

(Text labels are removed because of performance issues)

Implement absolutez flag for things & support flip flag

See merge request STJr/UltimateZoneBuilder!9

Added action to change the index of linedefs, things, sectors, and vertices. Can be accessed through a shortcut (not bound by default) or the menu of the respective mode (#903)