Bump minimum required GTK+ version

for the dialogs header bar handling.
Make KaPwDialog a proper child of GtkDialog

This gives us variants with and without header bar for free.
Pass on use-header-bar property to GtkDialog
Remove ka_pwdialog_create

and use ka_pwdialog_new instead
Retrieve app-menu via GResource

Not using a external glade file makes it simpler to run from the source
tree.
Retrieve prefs dialog via GResource

Not using a external glade file makes it simpler to run from the source
tree.
Retrieve main window via GResource

Not using a external glade file makes it simpler to run from the source
tree.
Update POTFILES.in
Make preferences a proper child of GtkDialog

This gives us variants with and without header bar for free.
Fix pkinit property names

This got broken by the move from GConf to GSettings
Don't try to set an empty property

Resulting from a c'n'p error we still set old_path resulting in ""
being treated as property.
Initially sync ticket properties

to properly reflect the current gsettings. This also got broken
during the GConf to GSettings switch.
Update copyright
Introduce ka_log_error_message_at_level

This allows us to log kerberos error message at different levels.
Sometimes they are expected and we don't want to issue a warning/
critical in order to not trap on G_DEBUG=fatal-warnings.
Don't log error about missing ticket cache at warning level

so we can continue to use fatal-warnings.
Silence status icon related deprecation warnings

We want to support this mode for some time for e.g. XFCE