gdk/wayland: Break only implicit grabs on wl_pointer.leave w/ pressed buttons (4.2)

The releasing of grabs while a button is pressed (e.g. after starting dnd, or dragging the window, or going to overview with a pressed button, etc...) was generalized here in !1879 (merged).

However we shouldn't break all grabs here. In the case of grabbing popups, compositors will still emit crossing events between client surfaces (e.g. popping up and selecting a menu item via press-drag-release), breaking all grabs here means inconsistent client state, that was #2746 (moved).

That was fixed in mutter, by essentially making implicit grabs owner_events=FALSE, however that breaks the mentioned use pattern entirely. Mutter is changing this behavior back, so GTK should handle these crossing events.

The grab that we are interested in breaking here is the implicit pointer one. Popups will be dismissed via other means if the compositor says their active grab needs breaking. This still leaves dnd/move/resize drags in one place, while not allowing #2746 (moved) to happen with popups.

(backport of !3625 (merged))