Redo this series of examples from 2013, and adapt it to modern
way of doing things. The biggest differences are that we use
a headerbar right from the start, and don't mention the app
menu.
Fixes: #2730
Setting up the icon and desktop file is a pretty central part
of making an application work, so we should do it for our example.
The fact that the examples are uninstalled makes this a little
more complicated.