Fortunately for us GtkTreeView still operates pretty
much exactly like in GTK3. Other than that, it's a
dense junction of all that we've done so far to port
other panels.
Add new class PpAuthenticationDialog which shows dialog
requesting username and password. The dialog has 3 parameters.
The first one is a text which will be shown to the user and
the second is username by which username entry will be prefilled.
The last parameter is parent window so thedialog is properly
decorated and positioned.
https://bugzilla.gnome.org/show_bug.cgi?id=683229
