Custom widget example: Use Gtk::StyleProperty

* docs/tutorial/C/index-in.docbook: Add the "Custom Style Properties"
subsection. In the "Contributing" chapter, update the filename
gtkmm-tutorial-in.xml to index-in.docbook.
* examples/book/custom/custom_widget/mywidget.[cc|h]: Use Gtk::StyleProperty
and Gtk::CssProvider::signal_parsing_error().