Use JSON for configuring the add-on settings

[Archomeda]

At first I thought that the current API would be good for configuring the optional settings window for add-ons. However, I feel that it's limited and not intuitive to work with. So I'm going to experiment to see if providing a JSON string is better instead.

This will most likely also change how the settings are stored for each add-on. Right now it's up to the add-on to store its settings (it doesn't matter if the loader is providing the GUI, or the add-on itself). But that doesn't feel intuitive either.

So the idea is to have 3 implementation options available to each add-on:

• GUI provided by the loader, settings saved/loaded by the loader (settings definitions provided through JSON by the add-on, add-on gets callbacks when settings changes)
• GUI provided by the add-on, settings saved/loaded by the add-on (add-on gets callback whenever the user wants to open the settings panel)
• No add-on settings

Note that only 1 of these options can be used at one time for each add-on individually, but each add-on can choose whatever they want.