Dialog: Make resizable handles easier to use

[tj.vantoll]

As is, the dialog touch handles are somewhat small. The corner handles (ne, se, nw, sw) give you a 7x7 target, and the side handles (n, s, e, w) give you a 2px wide/tall target to hit. I personally find these targets difficult to use, and they're certainly not touch friendly.

We should figure out a way of making the targets bigger without causing #9521 or #10543 to regress.