xwayland: add wlr_xwayland_surface_offer_focus()

In labwc, we have had trouble with XWayland windows using the Globally Active input model (see wlr_xwayland_icccm_input_model()). Under traditional X11, these windows do not expect to be given focus directly by the window manager; rather, the WM sends them a WM_TAKE_FOCUS message prompting the client to take focus voluntarily.

Currently, these clients are difficult to support with wlroots, because wlr_xwayland_surface_activate() assumes the client window will always accept the keyboard focus after being sent WM_TAKE_FOCUS. Some Globally Active client windows (e.g. panels/toolbars) don't want to be focused. It's useless at best to focus them, and might even make them misbehave. Others do need keyboard focus to be functional -- and there doesn't seem to be any reliable way to know this in advance.

Adding wlr_xwayland_surface_offer_focus() allows the compositor to send WM_TAKE_FOCUS to a client window supporting it and then see whether the client accepts or ignores 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().

This is entirely opt-in. A compositor that doesn't want to use the new function can continue to call wlr_xwayland_surface_activate() directly just as before, and need not make any use of the new focus_in signal.

The new focus_in signal might also be useful on its own, for the case where focus is allowed to move between windows belonging to the same PID. The compositor could then give a visual indication (e.g. titlebar color) of the spontaneous focus change.