Support a HdyKeypadBuffer
We need to make the buffer the Keypad stores the number in configurable. This will allow to use mlock'ed memory. Compare to what krb5-auth-dialog does with GtkEntryBuffer or even better GcrSecureEntryBuffer (https://developer.gnome.org/gcr/3.20/GcrSecureEntryBuffer.html). Otherwise secure PIN handling is not possible with that widget.