Menu filesystem access from search box, other fixes

Addresses #1324 and provides direct access to filesystem through search box
Adds clearing of previous search on menu close/search reset (repeated search showed empty results)

Addresses #1402 category sticking (stays highlighted while within)
Adds cursor memory (keyboard arrow into category, out, then back in)

Commits have been squashed here but are still available in iterations at [rjanja/1402-menu-fixes](https://github.com/rjanja/Cinnamon/commits/1402-menu-fixes).