layout: Implement border-spacing per CSS 2.1 § 17.6.1 and the legacy

`cellspacing` attribute per HTML5 § 14.3.9.

Table layout code has been refactored to push the spacing down to
rowgroups and rows; this will aid the implementation of
`border-collapse` as well.

This commit also fixes two nasty issues in table layout:

* In fixed layout, extra space would not be divided among columns that
  had auto width but had nonzero minimum width.

* In automatic layout, extra space would be distributed to constrained
  columns as well even if unconstrained columns with percentage equal to
  zero were present.

tests/ref/table_colspan_fixed_a.html

@@ -4,6 +4,7 @@
 table {
     width: 300px;
     table-layout: fixed;
+    border-spacing: 0;
 }
 td[colspan="2"] {
     background-color: blue;