The kde-neon extension reduces our boilerplate in snapcraft.yaml
and reduces the snap size considerably, as we do not need to ship
any library that is already included in the kde-frameworks-5-core18
snap itself.
This limits qelectrotech builds to the architectures supported
by kde-frameworks-5-core18, i.e. only amd64.
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.
