Better backend API

[pcercuei]

Modify/add functions:

• iio_context_add_device, which will create a iio_device and add it to the context;
• iio_device_add_channel, which will create a iio_channel and add it to the device;
• The functions iio_{context,device,channel}_add_attr to add attributes to contexts/devices/channels are now backend API
• iio_create_context_from_backend now takes a few extra parameters that will be used to create the iio_context.

With these changes, the XML backend was updated to be fully implemented on top of the public API (be it iio.h or iio-backend.h) and does not use iio-private.h anymore.

In theory, the local backend could do that too but it's a lot more work (due to how it currently creates the context).

The code should be considered very experimental, it was not thoroughly tested.

[buha]

@pcercuei you may merge this

[pcercuei]

Only people from ADI can merge 🙃

[pcercuei]

Gentle ping.