panel: Abstract the centered panel logic out into a ClutterLayoutManager

Since we want to use this in the overview as well, put it into centerLayout.js

Panel corners are disabled for now. They'll be added back eventually.

js/ui/centerLayout.js

@@ -0,0 +1,56 @@
+// -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
+
+const Lang = imports.lang;
+const Clutter = imports.gi.Clutter;
+
+const CenterLayout = new Lang.Class({
+    Name: 'CenterLayout',
+    Extends: Clutter.BoxLayout,
+
+    vfunc_allocate: function(container, box, flags) {
+        let rtl = container.get_text_direction() == Clutter.TextDirection.RTL;
+
+        let availWidth = box.x2 - box.x1;
+        let availHeight = box.y2 - box.y1;
+
+        // Assume that these are the first three widgets and they are all visible.
+        let [left, center, right] = container.get_children();
+
+        // Only support horizontal layouts for now.
+        let [leftMinWidth, leftNaturalWidth] = left.get_preferred_width(availWidth);
+        let [centerMinWidth, centerNaturalWidth] = center.get_preferred_width(availWidth);
+        let [rightMinWidth, rightNaturalWidth] = right.get_preferred_width(availWidth);
+
+        let sideWidth = (availWidth - centerNaturalWidth) / 2;
+
+        let childBox = new Clutter.ActorBox();
+        childBox.y1 = box.y1;
+        childBox.y2 = box.y1 + availHeight;
+
+        let leftSide = Math.min(Math.floor(sideWidth), leftNaturalWidth);
+        if (rtl) {
+            childBox.x1 = availWidth - leftSide;
+            childBox.x2 = availWidth;
+        } else {
+            childBox.x1 = 0;
+            childBox.x2 = leftSide;
+        }
+        childBox.x1 += box.x1;
+        left.allocate(childBox, flags);
+
+        childBox.x1 = box.x1 + Math.ceil(sideWidth);
+        childBox.x2 = childBox.x1 + centerNaturalWidth;
+        center.allocate(childBox, flags);
+
+        let rightSide = Math.min(Math.floor(sideWidth), rightNaturalWidth);
+        if (rtl) {
+            childBox.x1 = 0;
+            childBox.x2 = rightSide;
+        } else {
+            childBox.x1 = availWidth - rightSide;
+            childBox.x2 = availWidth;
+        }
+        childBox.x1 += box.x1;
+        right.allocate(childBox, flags);
+    }
+});