Xpra: Ticket #779: libfakexinerama cleanups

remove old global config for all displays since we don't use it
move all configs to directory pointed to by FAKEXINERAMA_DIR so we can move things out of the home dir (it's a bit messy having all these files)
enable this new cleaner setup if we find the right fakexinerama version, either using shared object versioning or just accessing a symbol via ctypes (without causing conflicts with the real lib!)