Fix setting "pressed" bitmap for wxToggleButton.

Move wxAnyButton::GetNormalState(), which allows wxToggleButton to override
what "normal" means for it, down to the platform-independent wxAnyButtonBase
class and use it now in wxGTK as well to correctly choose the pressed bitmap
for a toggle button in this state.

Closes #16771.

include/wx/anybutton.h

@@ -135,6 +135,13 @@ public:
         State_Max
     };
 
+    // return the current setting for the "normal" state of the button, it can
+    // be different from State_Normal for a wxToggleButton
+    virtual State GetNormalState() const
+    {
+        return State_Normal;
+    }
+
     // return true if this button shouldn't show the text label, either because
     // it doesn't have it or because it was explicitly disabled with wxBU_NOTEXT
     bool DontShowLabel() const