This allows building the master branch of QElectroTech with a simple
command:
$ flatpak-builder _build build-aux/flatpak/org.qelectrotech.QElectroTech.json
The Linux and Windows packaging recipes don't have any restrictions on
where they have to be located. Snapcraft is the strictest on this.
Moving this to build-aux/ means we can have all the packaging recipes in
one place.
Snapcraft can find its build recipes in 2 places:
* a snap/ directory at the top of the sources;
* a build-aux/snap/ directory, since Snapcraft 3.3;
This commit moves from the former to the latter, so that we can group
all the packaging recipes together.
