window: Only claim the sequence if keeping the grab

window: Only claim the sequence if keeping the grab

If the grab belongs elsewhere, the window won't claim the sequence right
away. The sequence may still be claimed afterwards when window dragging
starts, but simple clicks won't be consumed this way.

This makes it possible to close popovers when clicking on the title region,
while still permitting touch/button 1 interaction for every other purpose.

https://bugzilla.gnome.org/show_bug.cgi?id=743257