Apps: New Mousepad, Appfinder icons

New
- Xfce Appfinder
- Xfce Mousepad

Appfinder uses the `applications-all` and `applications-other` category
icons as base, hopefully tying to it the idea of "search for all applications".

Mousepad is based on the newer/future text editor icon from upstream elementary.

Both replace symlinks, which were used by many different apps/actions,
with unique icons. They're closer to the Xfce originals, and also use the
newer elementary gradients, colors, and rounded-square style.