Make config.json editor GUI

[pmer]

This will be a separate executable from the Driftwood engine executable.

Aimed toward Windows and macOS users (#133).

For the implementation, maybe check out wxWidgets since it is supposed to use native widgets.

[pmer]

One idea for its implementation would be to write a descriptor file that describes the structure and valid values for the config.json (similar to a JSON schema but with a few more details) and then have the editor create GUI widgets to match the descriptor. This way when the config file changes we only have to update the descriptor file and don't have to modify any GUI code directly, which should ease maintenance.

[seisatsu]

It sounds like we almost may as well implement it in the engine using our own widgets. I'm working on that sort of functionality and it'd be enough for something like this.

Otherwise we may want to write with QT4 so that our programs can be integrated with a custom version of Tiled later.

[pmer]

I like the within-engine solution.