Merge Johnny the GUI into the jumbo tree & build

I think Johnny the GUI will receive more attention from both (prospective) maintainers/developers and users if we merge it into jumbo. Have it as a configure option (maybe enabled by default, but skipping gracefully when not all required libraries are installed?) and have it built last (after all of the command-line tools have been built, so that a Johnny build failure wouldn't prevent access to any of the existing functionality of jumbo). This will also ensure that a user gets a compatible combination of versions of jumbo's command-line tools and Johnny (as long as we do maintain it).

To make it clear: the only merge I am proposing here is of the source trees and build procedures - have both projects combined into the same ./configure && make. Not any merge of the source code beyond the build scripts, etc. In fact, Johnny's source code should mostly remain in its own subdirectory in the merged tree (perhaps src/johnny or src/gui).

If there are no objections, then who's to work on it? Maybe @shinnok and/or @math07? :-)