Stylo: add -moz-border-*-colors support

In Gecko, we use double pointers to nsBorderColors to store -moz-border-*-colors.
We can simplify the implementation of computed value in Servo by using Option<Vec>.
As to passing computed values from Servo to Gecko, we might need to use some
binding functions (pre-added in the same Gecko bug, see Bug 1348173).

Note that we added -moz-border-*-colors as sub_properties of the 'border'
shorthand, so we can make the 'border' shorthand reset -moz-border-*-colors
(See Gecko Bug 482692). However, since they are Gecko only non-standard properties,
we should skip these sub_properties while doing unit testing in Servo. The test part
should be convered by Stylo/Gecko already.

A bit refactoring of replacing all ["top", "right", "bottom", "left"] with PHYSICAL_SIDES
is included in this patch, since we've already had PHYSICAL_SIDES for a while.

components/style/build_gecko.rs

@@ -624,6 +624,7 @@ mod bindings {
             .whitelisted_function("Gecko_.*");
         let structs_types = [
             "mozilla::css::URLValue",
+            "mozilla::Side",
             "RawGeckoAnimationPropertySegment",
             "RawGeckoComputedTiming",
             "RawGeckoDocument",