Add ObjectType trait to prelude

It's useful to have for the various API defined on it.

Author: sdroege

src/lib.rs

@@ -104,6 +104,7 @@ pub use object::{
     Object,
     ObjectExt,
     ObjectClass,
+    ObjectType,
     InitiallyUnowned,
     InitiallyUnownedClass,
     WeakRef,

src/prelude.rs

@@ -6,6 +6,7 @@ pub use {
     IsA,
     IsClassFor,
     ObjectExt,
+    ObjectType,
     StaticType,
     StaticVariantType,
     ToValue,