Add a new example app

Add a new example app

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.