This prevents the config and the user collection from being
backed up by snapd on update. If the user reverts qelectrotech,
the current collection will thus not be overwritten by the
possibly outdated backup.
Add a migration script that automatically transfers the user's .qet
directory to the new location.
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.
