libs/libnx/nxme: Add front-end, client, message handling needed for cursor support. Still actual cursor logic yet, just message handling.
graphics/nxmu and graphics/nxbe: Add back-end message handling needed for cursor support. No actual cursor logic yet, just message handling.
Squashed commit of the following:
libs/libxx: Fix some confusing in naming. If the directory is called libxx, then the library must be libxx.a (unless perhaps LIBCXX is selected).
libs/: Fix paths in moved library directories.
libs: Brute force move of libc, libnx, and libxx to libs. Cannot yet build it in that configuration.
