xwayland: use wlr_xwayland_surface_offer_focus()

xwayland: use wlr_xwayland_surface_offer_focus()

Offer focus by sending WM_TAKE_FOCUS to a client window supporting it.
The client may accept or ignore the offer. If it accepts, the surface will
emit a focus_in signal notifying the compositor that it has received focus.
The compositor should then call wlr_xwayland_surface_activate(surface, true).

This is a more compatible method of giving focus to windows using the
Globally Active input model (see wlr_xwayland_icccm_input_model()) than
calling wlr_xwayland_surface_activate() unconditionally, since there is no
reliable way to know in advance whether these windows want to be focused.

v2: add caution not to use view_offer_focus() directly
v3: remove obsolete comment