Work around for an apparent MinGW-w64 compiler (4.7.2-2) bug.

The public virtual inheritence of ResourceEditorDocument
 from Core::IDocument seems to be creating an invalid vtable.

It's now just public inheritence which is all that is needed.

The compiler in question was Ruben Van Boxem's 4.7.2-2.

Change-Id: I66f54ec04ce5aad847ac5bd6a4807fbd613c2d99
Reviewed-by: Daniel Teske <daniel.teske@digia.com>