Made TetheredBinding inherit from QQmlPropertyValueSource so that it can be used with syntax: TetheredBinding on property_name { ... }

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extra : amend_source : fd841f480badeebab5d49be75d854b146e2802f3
Added TetheredBinding-read_with_reassign test
Added TetheredBinding-circular test

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extra : amend_source : dd0997c02b8830dbcee6f7925cd308d7f5b52289
Added _set_from_prop
Fix deprecated use of string as slot reference

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extra : amend_source : 53ec746c52f1c75f757eb2360ac3db90ffa5a3a9
Fix segfault on key reuse after TetheredBinding destruction