loginDialog: s/button-press-event/key-press-event/

A bug got introduced when moving the login dialog away from modal
dialog, such that it listens for escape key presses in a mouse
event handler instead of a keyboard event handler.

This commit fixes that code to correctly listen for key-press-event
instead of button-press-event.

https://bugzilla.gnome.org/show_bug.cgi?id=702308