Always reset keyboard and pointer focus

If wl_pointer_set_focus or wl_keyboard_set_focus have been called before
a listener has been established for that seat and client combination,
the focus window will be set but the focus resource will be NULL.  This
changes these functions to always attempt to search for the relevant
focus resource, allowing the resource to be set by calling
wl_keyboard_set_focus and wl_pointer_set_focus again when a listener has
been established.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>