Thanks Adam Chainz for review and guidance.

Thanks Jaap Roes for the initial patch.

These values are duplicated in the super().

Regression in 374e6230.

Thanks Florian Apolloner for the report and analysis.

Unused (always True) after 3e43d24a.

Thanks Anton Gilgur for the report and review.

Area function, Distance function, and distance lookups now work with
geodetic coordinates on SpatiaLite.

* Avoid some unnecessary attribute lookups, e.g. access signals directly rather than from module
* Alias some repeat accesses inside the method to use the slightly faster local lookups
* Use tuple to iterate remaining kwargs as it's faster to construct
* Cache Field.get_default() to avoid running through all the logic on every call
* Use a cached list of the properties on the model class to avoid repeat isinstance() calls