seat: prevent xwayland from using incorrect keymap (#1816)

seat: prevent xwayland from using incorrect keymap (#1816)

With commit cafdcd8e, the keyboard from the keyboard group is set to the
seat when the active keyboard is destroyed, but this broke `wtype`
against XWayland surfaces. This is likely because XWayland caches
keycodes from the compositor and convert them with the last keymap
received from the compositor. So when we run `wtype a` over a XWayland
surface, the keycode for `a` is cached by XWayland, the keymap from the
keyboard group is sent to XWayland, then Xwayland converts the keycode
using the keymap from the keyboard group, not the one `wtype` set.

This commit fixes this by setting the keyboard from the keyboard group
to the seat when the keyboard focus is moved to a surface, not when the
active keyboard is destroyed.