Implement "Add unambiguous imports on the fly" option

[dima74]

changelog: Implement "Add unambiguous imports on the fly" option. You can turn it on in Settings | Editor | General | Auto Import. It can be useful e.g. when copying some code from a web site.

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Example:

Option in settings:

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I united AutoImportFix and AutoImportHintFix in order to get simpler code, and also in other languages there is no such division. Also now candidates are stored in AutoImportFix

[Kobzol]

Pretty cool! I suppose that this supersedes #7597?

class RsReferenceImporter : ReferenceImporter

👀 I see that I could have saved quite a few lines in my implementation :D

[dima74]

@Kobzol I think #7597 is needed too, because "unambiguous imports on the fly" option will be disabled by default (as in Java/Kotlin). Also even if user enable "unambiguous ..." option, #7597 will be useful when copying code with ambiguous name

[Undin]

Looks good in general but comments

[Undin]

bors r+

[bors]

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