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
Add a new example to the getting started part of the docs. The focus
of this example is on 'new stuff': GtkApplication, templates, settings,
gmenu, gaction, GtkStack, GtkHeaderBar, GtkSearchBar, GtkRevealer,
GtkListBox, GtkMenuButton, etc.
It is being developed in several steps. Each step is put in a separate
directory below examples/: application1, ..., application8. This is a
little repetitive, but lets us use the code of all examples in the
documentation.
