cursor: Allow leave/enter events within the same XDG toplevel

cursor: Allow leave/enter events within the same XDG toplevel

Attempting to open a GTK3 menu and activate a menu item in it,
using a single mouse motion (press-move-release), was broken due
to GTK apparently expecting to receive leave/enter events when the
cursor enters the menu (XDG popup).

To fix the issue, allow leave/enter events when the cursor is
moved between an XDG toplevel and popups of the same.

v2:
 - Use (struct view *) as proxy for toplevel in comparisons
 - Update seat->pressed.surface when entering/leaving popups
v3:
 - Go back to using get_toplevel() rather than (struct view *)