Fbs

The command is the usual `python -m fbs installer`. It generates a `.deb` file in the `target/` folder.

Previously, `fbs.platform` and `fbs_runtime.system` had basically the same purpose, and very similar functions. To remove this redundancy (and avoid possible confusion), they were now unified in the new module `fbs_runtime.platform`.

To migrate existing code, replace all references to `fbs.platform` and `fbs_runtime.system` by `fbs_runtime.platform`.

The number of this release should have been `0.2.1` as the next logical version after the previous one `0.2.0`. However PyPI did not allow publishing `0.2.1`, `0.2.2` or `0.2.3`, maybe because of some intermittent "Gateway 504" errors. So the number of this release is `0.2.4`.

It now also asks for the `version` setting (required on Mac), and gives better output.

* Don't require `clean` between two invocations of `installer`
* Clean up a backup file produced by `installer on Mac
* Add a default `mac_bundle_identifier` of `''` (empty) so first-time users can ignore it.

This flag generates a debug build that gives additional output when you run the frozen app on the command line.