Add a secure GtkEntryBuffer

Add a secure GtkEntryBuffer

We have a widget for password and passphrase entries, but we have no way
to handle the data securely. This is usually performed by a separate
GtkEntryBuffer—for instance, the one in GCR. While we have API for
setting a new entry buffer on GtkText, we don't have API for
GtkPasswordEntry, though, so the options are:

 - expose additional API for GtkPasswordEntry to allow setting a secure
   text buffer on the internal GtkText widget
 - provide a secure text buffer out of the box

Given that an insecure-by-default GtkPasswordEntry is basically
pointless, might as well have a secure buffer built in.

We don't really need to make the password entry buffer public out of the
box, but we can re-evaluate at a later date.

Fixes: #2403