enhancment: limit to horizontal or vertical movement of DMD window via shift/alt keys

[Rappelbox]

I'd like to suggest to implement a key function where the user can move the DMD window only vertically or horizontal.

I'm always struggling to find the perfect position without messing either X or Y coordinates. Usually i calculate the exact center horizontal position and depending on the DMD window i have to adjust it often vertically. but it always also messes the vertical position when moving the window. Holding the shift key could lock movement to one of the 2 directions. ;)

I'm sure others face this "issue" as well.