shellMountOperation: Implement new dialog design

Update the layout of the ShellMountPasswordDialog dialog according to
the new dialog design:

- Center-align all the elements of the dialog
- Align the work-spinner to the right (or left with RTL layouts) of the
password entry
- 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

@@ -95,6 +95,19 @@
   text-align: right;
 }
 
+.prompt-dialog-password-grid {
+  spacing-rows: 8px;
+  spacing-columns: 4px;
+
+  .prompt-dialog-password-entry {
+    width: auto;
+
+    // 4px (spacing) + 16px (spinner-width)
+    &:ltr { margin-left: 20px; }
+    &:rtl { margin-right: 20px; }
+  }
+}
+
 .prompt-dialog-password-layout {
   spacing: 8px;
 }
@@ -114,11 +127,6 @@
   color: $warning_color;
 }
 
-.prompt-dialog-grid {
-  spacing-rows: 15px;
-  spacing-columns: 1em;
-}
-
 
 /* Polkit Dialog */