extensionPrefs: Support extension updates

Now that we have support for extension updates in the shell, we
need some place to display the updates to the user.

As we are establishing the Extensions app as the primary way for
managing extensions, it's a natural place for that functionality.

Show which extensions have updates available, and offer a log out
button (so gnome-shell can apply the updates when logging back in).

https://gitlab.gnome.org/GNOME/gnome-shell/issues/1968

js/extensionPrefs/css/application.css

@@ -7,3 +7,5 @@
 .details-button.expanded:dir(rtl) image {
   -gtk-icon-transform: rotate(-0.25turn);
 }
+
+image.warning { color: @warning_color; }