Offer to search Google when no search results are returned

[jonstodle]

Adds a function on IQueryProvider to supply a "fallback result" which supplies a generic search with the provider. This makes it possible to add a setting specifying a desired fallback provider in the user settings later.

This closes #8.

[reinismu]

I feel like this kind of thing should be made differently. I propose that there should be property "IsGlobalProvider" and when search returns nothing or less than some amount then we could fill it in with global providers like google. Than any provider would have this option. Another thing is that not everyone will want this, so configuration must be in place for it.

[reinismu]

Don't think this should be here

[jonstodle]

Yes, this is a bit temporary (forgot to add a comment about that). Any suggestions on how to handle the cases where a provider is not global provider

[jonstodle]

I see your point about not every provider being a global provider. Do you think the IsGlobalProvider should be on IQueryProvider or some other interface?

[reinismu]

Maybe not a property, but extended IQueryProvider -> IGlobalQueryProvider. Tho it depends. Need more people opinion on this.

[jonstodle]

Yeah, that's more like what I was leaning towards too.

Let's get some more opinions, though 👍

[patriksvensson]

Nice initiative, but I think this PR strays from the original issue which was that the Google provider should allow you to search for things not suggested by the Google web service.

Personally I think that each provider should be responsible for whether or not to show fallback values, and not done globally.

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