tutorial: Improve type safety of property usage in GObject tutorial
tutorial: Improve type safety of property usage in GObject tutorial

This means that if you compile with `-Wswitch-enum`, the compiler will
warn you about properties which you’ve forgotten to handle in
`set_property()` or `get_property()`.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Helps: #1858

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib