Implement FromVariant for Variant

This is useful when combined with the container variant functions and
trait implementations from gtk-rs#651.

Implementing `ToVariant` would be useful as well, but isn't currently
possible: gtk-rs#678

src/variant.rs

@@ -725,6 +725,12 @@ where
  }
 }
 
+impl FromVariant for Variant {
+ fn from_variant(variant: &Variant) -> Option<Self> {
+ Some(variant.clone())
+ }
+}
+
 impl<K: StaticVariantType, V: StaticVariantType> StaticVariantType for DictEntry<K, V> {
  fn static_variant_type() -> Cow<'static, VariantTy> {
  let key_type = K::static_variant_type();