[feature] Value relation autocompleter match contains

[elpaso]

the default was to match from the beginning of the string ony.

A checkbox now allows
to activate the "contains" match
that will match also inside the string.

The old behavior is still the default to keep
backward compatibility.

[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit 62038a0)

[qgis-bot]

@elpaso

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