keyboard: Don't hide or show the keyboard immediately

Acting on each Show/Hide DBus call immediately may cause a lot of
jittery movement when Alt+Tabbing or even just switching tabs in
e.g. gnome-terminal.

To make the OSK feel sturdier, we wait a bit before actually showing
or hiding it so that we can coalesce tight sequences of Show/Hide
calls. I.e. the last call wins which means that we might end up not
doing anything.

https://bugzilla.gnome.org/show_bug.cgi?id=688646