ui: Add PadOsd

This is an implementation of the pad OSD that's been previously
present in gnome-settings-daemon. Since things are moving closer
to the compositor, it makes sense to have this implemented as shell
UI.

https://bugzilla.gnome.org/show_bug.cgi?id=771067

js/js-resources.gresource.xml

@@ -72,6 +72,7 @@
     <file>ui/osdMonitorLabeler.js</file>
     <file>ui/overview.js</file>
     <file>ui/overviewControls.js</file>
+    <file>ui/padOsd.js</file>
     <file>ui/panel.js</file>
     <file>ui/panelMenu.js</file>
     <file>ui/pointerWatcher.js</file>