workspace: Make room for a second geometry to keep track of

As we want to eventually track two geometries, we need to rename
our very plain "_x, _y, _width, _height". While we could just prefix
them, I think that stuffing them in an object makes more sense.

At the same time, make the variable and method name more descriptive
by adding such a prefix, as well as a bit of documentation.

https://bugzilla.gnome.org/show_bug.cgi?id=694469

js/ui/overviewControls.js

@@ -556,7 +556,7 @@ const ControlsManager = new Lang.Class({
         else
             geometry.x += thumbnailsWidth;
 
-        this.viewSelector.setWorkspacesGeometry(geometry);
+        this.viewSelector.setWorkspacesFullGeometry(geometry);
     },
 
     _setVisibility: function() {