[th/libnm-setting-cleanup]

Another small stab at unifying property handling in libnm.

We have property meta data (NMSettInfoProperty) for how to handle properties. On top of that, we have virtual functions in NMSettingClass. Also, keyfile has its own vtables.

How a property is handled is spread out to various places. Also, the function pointers in NMSettInfoProperty have non-obvious interactions. They got called at multiple times, and the caller had to take care about to call the right function. Try to improve that a bit.

Also, save the unnecessary calls for variant_type_for_gtype() all the time. We can cache the result.