Encapsulate the background image settings into a JSON object

Summary of the new feature/enhancement

As we're adding more settings to the JSON file, do we want to keep what's in each object flat or should we start introducing nested objects? I think having them be nested looks nicer and is easier to read (especially as the settings file gets really big). On the other hand, this might mean that we're forcing settings to look a certain way.

Consider this:

• Today:

{
    guid: ...,
    useAcrylic: ...,
    colorScheme: ...,
    backgroundImage: ...,
    backgroundImageOpacity: ...,
    backgroundImageStretchMode: ...,
    backgroundImageSetting4: ...,
    backgroundImageSetting5: ...
}

• Proposal:

{
    guid: ...,
    useAcrylic: ...,
    colorScheme: ...,
    backgroundImage: 
    {
        url: ...,
        opacity: ...,
        stretchMode: ...,
        setting4: ...,
        setting5: ...
    }
}

I think the proposal looks a lot nicer. I'm picking on backgroundImage from #853 but it definitely can apply to some other things like fontProperties (size, family, etc...) and devEnvironment (environmentVariables, startingDirectory, etc...).

Also, I think it's easier to see the benefit when there's a nasty looking settings file as follows:

{
    profileName: ...,
    backgroundImage: ...,
    guid: ...,
    backgroundImageOpacity: ...,
    useAcrylic: ...,
    backgroundImageStretchMode: ...,
    colorScheme: ...
}

The issue names no files, tests, or entry points. Start by locating the current settings schema and background-image handling, then review how related settings such as fontProperties and devEnvironment are represented. Done would require an agreed nesting design and corresponding implementation, but compatibility requirements and validation are not specified.