Support UI scaling in the icon view

The icon view previously was not taking into account the UI scale factor when reading _NET_WORKAREA directly. The WM of course doesn't know or care about UI scaling, so _NET_WORKAREA is in device units, not "application" units. When reading the workarea or screen size, GDK automatically converts to device units, so those code paths were fine.

As a bonus, this fixes a typo I made a month or so ago when changing the icon redraw code. I accidentally passed the y coord instead of height to gtk_widget_queue_draw_area() when adding a new item to the view. This would work for most single-icon redraws except the top row. Not sure why I didn't notice that before.