Fix bitmap accessor for the buttons in wxQt

Return the bitmap previously set with SetBitmap().

This fixes unit tests using GetBitmap().

Closes https://github.com/wxWidgets/wxWidgets/pull/1035

src/qt/anybutton.cpp

@@ -58,6 +58,8 @@ void wxAnyButton::QtSetBitmap( const wxBitmap &bitmap )
     QPixmap *pixmap = bitmap.GetHandle();
     m_qtPushButton->setIcon( QIcon( *pixmap  ));
     m_qtPushButton->setIconSize( pixmap->rect().size() );
+
+    m_bitmap = bitmap;
 }
 
 void wxAnyButton::SetLabel( const wxString &label )
@@ -70,6 +72,11 @@ QWidget *wxAnyButton::GetHandle() const
     return m_qtPushButton;
 }
 
+wxBitmap wxAnyButton::DoGetBitmap(State state) const
+{
+    return state == State_Normal ? m_bitmap : wxNullBitmap;
+}
+
 void wxAnyButton::DoSetBitmap(const wxBitmap& bitmap, State which)
 {
     switch ( which )