Implement ToVariant and FromVariant for Variant

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

BREAKING CHANGE: This remove `impl<T: ToVariant> From<T> for Variant`,
which made it impossible to implement `ToVariant` since it conflicts
with the standard library `impl<T> From<T> for T`:
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();