Added TODO re. icon not appearing in body of NSIS binary installer.
License no longer included in user guide.
Removed build_user_guide.tcl script as no longer required.
Related changes in CMakeLists.txt; also reflecting in CMakeLists.txt
that LICENSE is no longer incorporated into user guide.
Reflected removal of LICENSE from user guide in user_guide/index.rst.
Added brief copyright notice at top of LICENSE file.
Added TODO re. whether to use wxAboutDialog or custom AboutDialog.