Revert "combobox: remove superfluous GTK+ grabs"

Revert "combobox: remove superfluous GTK+ grabs"

This reverts commit 46b4e5d38877197503f802380e5c9a469218f99b.

Apparently these weren’t so redundant, because their removal introduced
at least 2 bugs in list-mode ComboBoxes: failure to close the popup upon
clicking out of it, and failing to receive mouse input in the popup of a
CB in a modal window. It may also have caused (even more!) stuck grabs.

https://bugzilla.gnome.org/show_bug.cgi?id=738387
https://bugzilla.gnome.org/show_bug.cgi?id=776793