Use points for font sizes

Do a basic job of converting font sizes from pixels to points, so they
will scale will the global GNOME scale factor. Some other sizes that are
clearly related to the font sizes are changed to ems, but no comprehensive
attempt is made to get rid of px units.

https://bugzilla.gnome.org/show_bug.cgi?id=636868

js/ui/overview.js

@@ -439,7 +439,7 @@ Overview.prototype = {
         let primary = global.get_primary_monitor();
         let rtl = (St.Widget.get_default_direction () == St.TextDirection.RTL);
 
-        let contentY = Panel.PANEL_HEIGHT;
+        let contentY = Main.panel.actor.height;
         let contentHeight = primary.height - contentY - Main.messageTray.actor.height;
 
         this._group.set_position(primary.x, primary.y);