build: Move the Snap recipes

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.

build-aux/snap/local/launchers/qelectrotech-launch

@@ -0,0 +1,6 @@
+#!/bin/bash
+
+mkdir -p "$HOME/.qet"
+ln -snf "$SNAP/bin/DXFtoQET" "$HOME/.qet/DXFtoQET"
+
+exec "${@}"