add functionality to create source packages

there needs to be a way to "tar up" a project's source tree to create a source package. that makes is possible to easily create minimal source distributions while keeping auxiliary files in the source tree (e.g., the build tree and maintainer-only scripts).
automake provides this in the form of the "dist" target (plus a "distcheck" target that instantly unpacks and builds the archive to verify that it is self-contained), cmake via the "package_source" target (only if cpack installed, if i gather it correctly).

the scope of the single target should be the chosen top-level project, including all sub-projects it aggregates.

implementation-wise, this would probably use the same api that the qtc project explorer uses to get a complete list of files, and feed that to the already existing archiver module.