A Gtk.Application subclass to handle the application's initialization
and integration with the GNOME stack. It implements the do_startup and
do_activate methods and is responsible for the application's menus,
icons, title and lifetime.
It's good to have to code comply to a style guide; we go for PEP8
because it's the most used one. Checking for conformance should be done
using the `flake8` tool and the configuration given in this commit.
E402 and E501 are ignored because we don't care (that much) about line
length, and GObject Introspection requires us to import things as we
currently do.
piper/piper.py has not been fixed as it is going to be removed
eventually, when all its relevant bits have found a place in the new
code. It is therefore also ignored in the checking.
As discussed in #5, this enables better integration with the GNOME stack
through Meson's GNOME module. This will allow us to use GSchemas,
AppData and other GNOME technologies much easier. It will also simplify
building Flatpaks of Piper.
