keyboard: Fix grabs not working when showing the dialog

We couldn't override gnome-shell's keybindings without having a working
grab, but the grab was only started when clicking the "edit" button when
editing a custom shortcut, or *after* receiving the first key press event.

To fix that problem, we need to grab the keyboard once we've shown the
dialog itself, but not in the ->map vfunc, otherwise it will block the
dialog from showing up. We set up a short timeout instead. Hopefully
this isn't too fragile.

https://bugzilla.gnome.org/show_bug.cgi?id=769314