Return empty bitmaps from wxButton if not explicitly set in wxMSW

Do this for consistency with the other ports and because this seems more
useful anyhow.

Update the documentation to make this behaviour more clear and document
this change as a (minor) incompatibility in wxMSW.

Also add more unit tests to check for this behaviour. Note this also
fixes the problem with the unit test added in the grandparent commit
under MSW.

docs/changes.txt

@@ -129,6 +129,10 @@ Changes in behaviour not resulting in compilation errors
 - wxSpinCtrl::SetValue(wxString) overload doesn't generate any events with
   wxMSW, which was already the documented behaviour.
 
+- wxButton::GetBitmap{Current,Disabled,Focus,Pressed}() only return valid
+  bitmaps in wxMSW if the corresponding Set had been called before, as in the
+  other ports, instead of returning the normal bitmap as fallback in this case.
+
 
 Changes in behaviour which may result in build errors
 -----------------------------------------------------