glib: Add a method to Class<T> to get the wrapped C struct

This makes it easier/safer to implement `IsSubclassable`. This does not
(yet) rewrite all the relevant code to use this method.

Author: ids1024

glib/src/object.rs

@@ -2513,6 +2513,11 @@ impl<T: ObjectType> Class<T> {
             }
         }
     }
+
+    /// Gets the contained type struct
+    pub fn inner(&mut self) -> &mut T::GlibClassType {
+        &mut self.0
+    }
 }
 
 unsafe impl<T: ObjectType> Send for Class<T> { }