Revert "Stop relying on the launcher binary for traditional activation"

This reverts commit 65a815f5bc3e9e8d0558c1d8ecf3f936d5c1568b.
package.js has too many hacks already, let's not muddle the
water more between the launcher and the actual binary.
Use appctl to launch the application from legacy environments

The launcher binary is not part of the upstream binary package,
so use "appctl" instead.
Note that no runtime dependency is added for it, it should be
pulled in by environments that don't use GIO or Qt to launch
apps.
rpm: build rpms in a clean environment

Prevents paths and library sonames from jhbuild to creep in the
final RPM.
I also looked into mock, but it's quite Fedora specific, while
this should allow one to build RPMs that work everywhere - well,
if you ignore glibc ABI issues...