LibWeb: Add ComputedCSSSstyleDeclaration and support 2 properties :^)

getComputedStyle(element) now returns a ComputedCSSStyleDeclaration
object, which is a live view of the computed style of a given element.

This works by ComputedCSSStyleDeclaration being a wrapper around an
element pointer. When you ask it for a CSS property, it gets the latest
computed style values from the element and returns them as a
CSS::StyleProperty object.

This first cut adds support for computed 'color' and 'display'.

In case the element doesn't have a corresponding node in the layout
tree, we fall back to using specified style instead. This is achieved by
performing an on-the-fly style resolution for the individual element and
then grabbing the requested property from that resolved style.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -17,6 +17,7 @@ set(SOURCES
     CSS/CSSStyleDeclaration.cpp
     CSS/CSSStyleRule.cpp
     CSS/CSSStyleSheet.cpp
+    CSS/ComputedCSSStyleDeclaration.cpp
     CSS/DefaultStyleSheetSource.cpp
     CSS/Length.cpp
     CSS/MediaQueryList.cpp