Move definition of animatable for shorthands to Shorthand object

By moving this definition to the Shorthand object we can more easily
re-use it in subsequent patches in this series.

components/style/properties/data.py

@@ -234,6 +234,16 @@ class Shorthand(object):
         self.allowed_in_keyframe_block = allowed_in_keyframe_block \
             and allowed_in_keyframe_block != "False"
 
+    def get_animatable(self):
+        animatable = False
+        for sub in self.sub_properties:
+            if sub.animatable:
+                animatable = True
+                break
+        return animatable
+
+    animatable = property(get_animatable)
+
 
 class Method(object):
     def __init__(self, name, return_type=None, arg_types=None, is_mut=False):

components/style/properties/helpers/animated_properties.mako.rs

@@ -195,21 +195,11 @@ impl TransitionProperty {
 #[cfg(feature = "gecko")]
 pub fn nscsspropertyid_is_animatable(property: nsCSSPropertyID) -> bool {
     match property {
-        % for prop in data.longhands:
+        % for prop in data.longhands + data.shorthands_except_all():
             % if prop.animatable:
                 ${helpers.to_nscsspropertyid(prop.ident)} => true,
             % endif
         % endfor
-        % for prop in data.shorthands_except_all():
-            <%
-                animatable = "false"
-                for sub in prop.sub_properties:
-                    if sub.animatable:
-                        animatable = "true"
-                        break
-            %>
-            ${helpers.to_nscsspropertyid(prop.ident)} => ${animatable},
-        % endfor
         _ => false
     }
 }