[th/platform-move] move more files from src/core/platform to src/libnm-platform/

The platform code should be an independent module, under src/libnm-platform.

It almost already is, in particular it's mostly independent from code under src/core/. Where it currently is not, this needs fixing (moving code around).

Also, libnm-platform should be independent of src/libnm-core-{public,intern}. The reason is that the majority of libnm-core is about settings, and platform code should not be about that. By keeping the dependencies separate, the code is supposed to be more modular. However, we also use several enums from libnm-core, so doing this is slightly cumbersome.

Still, here another patch that moves some more of the platform files.