Add experimental RGUI keyboard search feature.

Type '/' to start search, then type search string (case sensitive for
now due to strcasestr being GNU ext). When done, type return.
Merge branch 'master' into rgui-search
Refactor keyboard event callbacks.

Go through input/keyboard_line.c to better hide internal logic
regarding keyboard line input.
Update griffin.
Fix CXX_BUILD.
Fix menu_search_callback.

str can be empty and non-NULL.
Use strcasestr.

Since it's a GNU extension, move to strcasestr.h.
Add strcasestr check to quickbuild.
Prioritize matches with first chars.
Merge pull request #410 from libretro/rgui-search

Add experimental RGUI keyboard search feature.