panel: Switch to fully dynamic layout

There was lots of fixed positioning in the Panel; now it is completely
dynamic, and width/height is driven from main.js.  We still have a
global constant Panel.PANEL_HEIGHT, but this is a big step towards
eliminating it.

Also, this avoids overdraw in the "way too many tray icons" case.  The
clock will shift left.

js/ui/overview.js

@@ -127,8 +127,6 @@ Overview.prototype = {
         this._transparentBackground.lower_bottom();
         this._paneContainer.lower_bottom();
 
-        this._repositionChildren();
-
         this._workspaces = null;
     },
 
@@ -148,7 +146,7 @@ Overview.prototype = {
         }
     },
 
-    _repositionChildren: function () {
+    relayout: function () {
         let global = Shell.Global.get();
 
         let contentHeight = global.screen_height - Panel.PANEL_HEIGHT;