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/tglbtn.h

@@ -36,6 +36,12 @@ public:
     virtual void SetValue(bool state) = 0;
     virtual bool GetValue() const = 0;
 
+    // The current "normal" state for the toggle button depends upon its value.
+    virtual State GetNormalState() const wxOVERRIDE
+    {
+        return GetValue() ? State_Pressed : State_Normal;
+    }
+
     void UpdateWindowUI(long flags) wxOVERRIDE
     {
         wxControl::UpdateWindowUI(flags);