Fixes relation reference widget when chain filter option is activated, fixes #16903

[pblottiere]

Description

This PR fixes an issue regarding the relation reference widget : https://issues.qgis.org/issues/16903

When the Chain Filter option is activated, a filtering is now resolved on the fly with cached values in order to provide only filtering values (in combo boxes) leading to actual features.

Some tests have been added to validate the behavior.

See the initial discussion : qwat/QWAT#186

Checklist

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• I have read the QGIS Coding Standards and this PR complies with them
• This PR passes all existing unit tests (test results will be reported by travis-ci after opening this PR)
• New unit tests have been added for core changes
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