xwayland: add about_to_map signal

For XWayland surfaces that start maximized, it's best to send an initial Configure event to set the size of the surface before mapping it. This reduces visual glitches since the application sees the correct maximized size when performing its initial layout and drawing.

wlroots surfaces emit their first "map" event after the XWayland window has already been mapped and the first frame has been drawn & committed. This is too late to send the initial Configure event.

So, add a new "about_to_map" event which is emitted immediately before telling XWayland to map the window. Compositors can connect to this event to send the initial Configure event to an XWayland app based on its requested maximized (or fullscreen) state.

Compositors should not place anything visually on the screen at this point but rather wait until the "map" event as before.