accessibility: Add pointer accessibility support

Adds the UI part for the pointer accessibility features.

The various timeouts running are notified using a pie-timer showing
under the pointer.

For dwell-click type selection, we use a drop-down menu. Users can
use the dwell-click to select the next type of dwell click to be
emitted.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/474

js/ui/panel.js

@@ -817,6 +817,7 @@ const PANEL_ITEM_IMPLEMENTATIONS = {
     'dateMenu': imports.ui.dateMenu.DateMenuButton,
     'a11y': imports.ui.status.accessibility.ATIndicator,
     'keyboard': imports.ui.status.keyboard.InputSourceIndicator,
+    'dwellClick': imports.ui.status.dwellClick.DwellClickIndicator,
 };
 
 var Panel = GObject.registerClass(