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.

[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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