Dimventory Fixup

Corrects the dimension specific add/remove button, and the main button, so that they do not do very weird things with scaling and positioning, such as stretching around and being clamped to the minimap, or resetting to topleft on brief-right-click and starting to play 'dodge the mouse' or sticking to the mouse. Also adjusted offset position from mouse while dragging, so that buttons -should- remain relatively centered on the mouse when dragging.