Increased performance when deleting things in Things mode, linedefs in...

Increased performance when deleting things in Things mode, linedefs in Linedefs mode and sectors in Sectors mode.
Increased performance when canceling Edit mode after pasting.
"Export to Wavefront .obj" function: current selection is now converted to sectors before running the function (previously it would export selected sectors when there were sectors selected, or the whole map when no sectors were selected).