keyring: Implement new dialog design

Replace the ClutterGridLayout of the keyring dialog with a vertical
StBoxLayout based layout according to the new dialog design:

- Center-align all the elements of the dialog
- Remove the work-spinner because requests usually finish fast enough
- Show the entry-labels as hint text of the entry

See https://gitlab.gnome.org/GNOME/gnome-shell/issues/1343

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/942

data/theme/gnome-shell-sass/widgets/_dialogs.scss

@@ -170,8 +170,3 @@
   spacing-rows: 15px;
   spacing-columns: 1em;
 }
-
-.keyring-dialog-control-table {
-  spacing-rows: 15px;
-  spacing-columns: 1em;
-}