keyboard: Don't grab the mouse pointer

In a0a15588, we starting using a separate shortcut editor window, which
was doing its own capture instead of using the GtkCellRendererAccel. But
this started grabbing both keyboard and pointer, making it impossible to
cancel captures using the pointer.

We now only grab keyboard keys, making the pointer usable again.

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