Use XOR operation to toggle the state of check box in wxPG.

Minor optimization by using single XOR operation instead of separate AND, OR operations to toggle the state of wxSimpleCheckBox.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76886 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775