Extract Method qtro_enum_signature_impl() from qtro_enum_signature()

This mirrors how other qtro_ functions are implemented, and drags the
template-argument-independent parts out of the function template
("SCARY").

Found (as pre-existing) in API-review.

Amends c96a4193e5d4d46804e8a11fad3b0e95d02b037b (5.12.4).

Can't pick to released branches, as it creates a new exported symbol.

Pick-to: 6.12
Change-Id: I2894c56b0e27de60c6740c5afa453fbc8b2e0ac6
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>