Let wayland-server send modifier events for focus

Let wayland-server send modifier events for focus

When the focus changes, wl_keyboard_set_focus (and, as an added bonus,
wl_pointer_set_focus) will now send wl_keyboard::modifier events for us
if we store the modifier state in the right place, so we don't have to
worry about that anymore.

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