Experimental right-alignment for hover layer tools

This is not necessarily a permanent commit, but one I'm going to try for
a few days to see how I like it.  One of the problems with PD's current
"hover options" on the Layer box is that they can easily be accidentally
hit when rapidly clicking from the canvas to the layer menu.

To prevent this, I'm considering right-aligning the hover menu.  This
provides a few benefits: it provides more empty clickable space on the
left-side of the layer toolbox (the side nearest the image canvas),
preventing accidental clicks.  It also makes the "visibility" toggle
adhere better to Fitt's Law, by sticking it on the far right-side of the
layers panel.

That said, the new arrangement looks funny to me, but it could just be
because I'm accustomed to the old alignment.  Will revisit in a few days
after having time to test it.
Re-merge missing high-DPI fixes

No idea what happened here; it's like a sync on my Win 7 PC failed
halfway through, because only half of my high-DPI commit is showing up
in my primary Win7 PC!  Damn GitHub for Windows...