Philip Withnall authored Mar 15, 2019 and Richard Hughes committed Jul 17, 2019

This adds some utility functions for accessing a metered data scheduler,
exposed in the public API if GS_ENABLE_EXPERIMENTAL_MOGWAI is defined
before the library headers are included.

Using this unstable API from out of tree plugins is not supported.

Support will be added to some of the in-tree plugins in following
commits, allowing them to schedule their large downloads using the
system scheduler, in accordance with the user’s metered data policy.

This adds an optional dependency on libmogwai-schedule-client.

It will be the responsibility of individual plugins to ensure that large
downloads (roughly, >100KB) are scheduled using the scheduler. Smaller
downloads don’t have to be, as they consume negligible bandwidth.

For now, remove use of the download-updates setting, as the policy it
enforced is implemented more flexibly in the download scheduler daemon.

Signed-off-by: Philip Withnall <withnall@endlessm.com>