Expose OS-wide dark style preference

[cassidyjames]

Will fix #276. Right now this is just a method that returns a bool, but I imagine it'd be more useful to be able to bind to this. Updated to be a property. Not sure where it would belong though. Feedback welcome.

[worldofpeace]

If the GSetting schema that contains the setting does not

• exist, this will not connect and cb will never be called.

But because the GSettings Schema is provided by switchboard-plug-pantheon-shell it creates a circular cycle between granite.

It shouldn't cause a failure because of the precaution, but it would be a good practice to avoid that entirely.

[cassidyjames]

@worldofpeace yeah I suppose in this case Granite could create the GSettings Schema itself?

[donadigo]

@cassidyjames in my opinion ideally for such a setting, no non-core / exclusive package should manage how this setting is created. The setting should be created and built-in on an OS level (e.g. at install). This ensures that we don't add any unnecessary dependencies.

[worldofpeace]

@worldofpeace yeah I suppose in this case Granite could create the GSettings Schema itself?

There's a similar problem with #242 and @danrabbit said that didn't belong in granite either.
Seemed we mostly agreed on there being an elementary-desktop-schemas.

I think we should go with that.

[cassidyjames]

Closing in favor of #319