Add Servo_GetStyleFoo functions to get style structs from GeckoComputedValues.

While we're here we also:

* remove any code conditional on style_struct.gecko_ffi_name, since all
  style structs now do have a corresponding Geckos struct

* add new UIReset and XUL style structs, so that all Gecko structs are
  now present (apart from Variables, which is special)

components/style/properties/longhand/box.mako.rs

@@ -7,7 +7,7 @@
 
 <% data.new_style_struct("Box",
                          inherited=False,
-                         gecko_ffi_name="nsStyleDisplay",
+                         gecko_name="Display",
                          additional_methods=[Method("transition_count", "usize")]) %>
 
 // TODO(SimonSapin): don't parse `inline-table`, since we don't support it