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/msw/anybutton.h

@@ -36,7 +36,6 @@ public:
     virtual WXLRESULT MSWWindowProc(WXUINT nMsg, WXWPARAM wParam, WXLPARAM lParam);
 
     virtual bool MSWOnDraw(WXDRAWITEMSTRUCT *item);
-    virtual State GetNormalState() const { return State_Normal; }
 
     // returns true if the platform should explicitly apply a theme border
     virtual bool CanApplyThemeBorder() const { return false; }