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/tglbtn.h

@@ -46,8 +46,6 @@ public:
     virtual bool MSWCommand(WXUINT param, WXWORD id);
     virtual void Command(wxCommandEvent& event);
 
-    virtual State GetNormalState() const;
-
     // returns true if the platform should explicitly apply a theme border
     virtual bool CanApplyThemeBorder() const { return false; }