Followup issues from always visible arrows

Some followup issues from !2335 (merged), I tested it on @rmader's Pinebook:

• When DnDing an icon over the preview area near the edge, it's unexpected that it switches pages. I'd keep that as a drop target, but not switch pages on hover anywhere but the actual edge of the screen
• The icons from adjacent pages moving in from the sides during DnD is a bit disorienting because there's no clear distinction between pages, since the spacing looks the same as the regular spacing between app grid columns. I'd move the icons out by about 100px less, so the gap is more clearly different from the one between columns.