Support fixed position children

When doing layout, check for whether x/y have been explicitly
specified, and if so honor those positions.

https://bugzilla.gnome.org/show_bug.cgi?id=598462

tests/interactive/box-layout.js

@@ -44,6 +44,12 @@ colored_boxes.add(new St.Label({ text: "Default",
                                  style: 'border: 1px solid #aaaaaa; '
                                         + 'background: #cceeff' }));
 
+b2.add(new St.Label({ x: 50,
+                      y: 50,
+                      text: "Fixed",
+                      style: 'border: 1px solid #aaaaaa;'
+                             + 'background: #ffffcc' }));
+
 ////////////////////////////////////////////////////////////////////////////////
 
 function createCollapsableBox(width) {