Add tests and a Sizable trait for ContextElement.

src/renderer/context_element.rs

@@ -22,6 +22,7 @@ pub trait ContextElement:
     + FromContextElement
     + IntoRcIce
     + Castable
+    + Sizable
 {
 }
 
@@ -63,6 +64,10 @@ pub trait Castable {
     fn cast_to_type<'a>(&'a self, target: &str) -> Option<IceResult<'a>>;
 }
 
+pub trait Sizable {
+    fn get_size<'a>(&'a self) -> Option<IceResult<'a>>;
+}
+
 pub trait CastToAny {
     fn to_any(&self) -> &dyn Any;
 }