XLib halfkey implementation

 Running xhk detaches the keyboard, and intercepts all input; processing it for
 space bar pushes as it goes. The space bar acts as a modifier if it is held at
 the same time as a standard letter. Modifying acts to mirror the input such
 that key presses are mirrored down the middle of the home row.
 Pressing space, and then releasing will still provide a single space character.