[th/move-src-core-dir] all: move "src/" directory to "src/core/"

Currently "src/" mostly contains the source code of the daemon. I say mostly, because that is not true, there are also the device, settings, wwan, ppp plugin and the initrd generator.

Also we have source code under libnm-core, libnm, clients, and shared directories. That is all confusing.

We should have one "src" directory, that contains subdirectories. Those subdirectories should contain individual parts (libraries or applications), that possibly have dependencies on other subdirectories.

As the name "src/" is already taken, that prevents any sensible restructuring of the code.

As a first step, move "src/" to "src/core/". This gives space to reorganize the code better by moving individual components into "src/".

For inspiration, look at systemd's "src/" directory.