keyboard: Fix subkeys handling

We can't pushModal() when showing the subkeys popup because that will
cause the application to lose focus and thus we get a Hide() call for
the whole OSK.

Instead, capture events on the main OSK actor while the subkeys popup
is shown so that we can both prevent events from reaching the main
keys but also cancel the subkeys if the user clicks away in the OSK.

https://bugzilla.gnome.org/show_bug.cgi?id=674955