Add experimental RGUI keyboard search feature.

Don't merge this yet. It's kinda ugly and experimental. It's my idea for how keyboard search can be done with RGUI, or typing in general.

Type '/' to start search when in RGUI, then type search string (case sensitive for
now due to strcasestr being GNU ext). When done, type return and the selection ptr should get updated.

It searches the whole string for a match with strstr, so it's O(n). The upside is that you can search for strings mid-filename.

When you're typing, all hotkeys are completely disabled until you hit return, so that part should be covered nicely.