layout: Implement text-transform.

The Unicode awareness of `text-transform` is implemented as well as
possible given the Rust standard library's Unicode support. In
particular, the notion of an alphabetic character is used instead of a
letter.

Gecko has a subclass of text run to handle text transforms, but I
implemented this in a simpler way.

components/style/properties/mod.rs.mako

@@ -1185,6 +1185,9 @@ pub mod longhands {
 
     ${single_keyword("white-space", "normal pre nowrap")}
 
+    // TODO(pcwalton): `full-width`
+    ${single_keyword("text-transform", "none capitalize uppercase lowercase")}
+
     // CSS 2.1, Section 17 - Tables
     ${new_style_struct("Table", is_inherited=False)}