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.