Property mapping improvement

In the 7.x version of the module, we had the ability to target sub-fields (like the individual elements of a URL or Address field) using the property picker. In 8.x, that is possible through the "Properties Extended" mapping field type, but it's awkward to use, with a noisy auto-complete and a complicated input switcher.

This code pulls in a lot of the data-access logic from the Properties Extended, but borrows the UX from the Mailchimp Lists submodule to identify sub-properties and make them available for selection.

It looks like a lot of code, but most of the work is just to reduce redundancy by moving a lot of the code from the Properties Extended mapping field plugin into the Properties plugin, and then making Properties Extended inherit from Properties.