LibWeb: Some improvements to absolute positioning

Absolutely positioned blocks now register themselves with their
containing block (and note that the containing block of an absolutely
positioned box is the nearest non-statically positioned block ancestor
or the ICB as fallback.)

Containing blocks then drive the layout of their tracked absolutely
positioned descendants as a separate layout pass.

This is very far from perfect but the general direction seems good.

Base/home/anon/www/position-absolute-from-edges.html

@@ -0,0 +1,48 @@
+<style>
+#container {
+    position: absolute;
+    width: 500px;
+    height: 400px;
+    border: 1px solid black;
+}
+#red {
+    position: absolute;
+    left: 20px;
+    top: 20px;
+    background: red;
+    width: 100px;
+    height: 100px;
+    border: 10px solid gray;
+}
+#green {
+    position: absolute;
+    right: 20px;
+    top: 20px;
+    background: green;
+    width: 100px;
+    height: 100px;
+    margin: 50px;
+}
+#blue {
+    position: absolute;
+    left: 20px;
+    bottom: 20px;
+    background: blue;
+    width: 100px;
+    height: 100px;
+}
+#yellow {
+    position: absolute;
+    right: 20px;
+    bottom: 20px;
+    background: yellow;
+    width: 100px;
+    height: 100px;
+}
+</style>
+<div id=container>
+    <div id=red></div>
+    <div id=green></div>
+    <div id=blue></div>
+    <div id=yellow></div>
+</div>