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.

components/script/dom/htmltableelement.rs

@@ -26,6 +26,7 @@ pub struct HTMLTableElement {
     htmlelement: HTMLElement,
     background_color: Cell<Option<RGBA>>,
     border: Cell<Option<u32>>,
+    cellspacing: Cell<Option<u32>>,
     width: Cell<LengthOrPercentageOrAuto>,
 }
 
@@ -45,6 +46,7 @@ impl HTMLTableElement {
                                                     document),
             background_color: Cell::new(None),
             border: Cell::new(None),
+            cellspacing: Cell::new(None),
             width: Cell::new(LengthOrPercentageOrAuto::Auto),
         }
     }
@@ -94,6 +96,7 @@ impl<'a> HTMLTableElementMethods for JSRef<'a, HTMLTableElement> {
 pub trait HTMLTableElementHelpers {
     fn get_background_color(&self) -> Option<RGBA>;
     fn get_border(&self) -> Option<u32>;
+    fn get_cellspacing(&self) -> Option<u32>;
     fn get_width(&self) -> LengthOrPercentageOrAuto;
 }
 
@@ -106,6 +109,10 @@ impl HTMLTableElementHelpers for HTMLTableElement {
         self.border.get()
     }
 
+    fn get_cellspacing(&self) -> Option<u32> {
+        self.cellspacing.get()
+    }
+
     fn get_width(&self) -> LengthOrPercentageOrAuto {
         self.width.get()
     }
@@ -132,6 +139,9 @@ impl<'a> VirtualMethods for JSRef<'a, HTMLTableElement> {
                                                                      .as_slice()
                                                                      .chars()).unwrap_or(1)))
             }
+            &atom!("cellspacing") => {
+                self.cellspacing.set(str::parse_unsigned_integer(attr.value().as_slice().chars()))
+            }
             &atom!("width") => self.width.set(str::parse_length(attr.value().as_slice())),
             _ => ()
         }
@@ -145,6 +155,7 @@ impl<'a> VirtualMethods for JSRef<'a, HTMLTableElement> {
         match attr.local_name() {
             &atom!("bgcolor") => self.background_color.set(None),
             &atom!("border") => self.border.set(None),
+            &atom!("cellspacing") => self.cellspacing.set(None),
             &atom!("width") => self.width.set(LengthOrPercentageOrAuto::Auto),
             _ => ()
         }