Add some callback functionality, so that test code can inject/change
settings before they are processed.
Will be used in an actual test in the following commit.
This introduces some more conceptual changes to Shaman. The most important
one is that there is no longer a "checkout ID", but a "checkout path".
The Shaman client can request any subpath of the checkout directory,
so that it can handle things like project- or scene-specific prefixes.
A boolean provides less context to the setting, so it's not as easy to
understand. However, in this case the simple case will have `is_twoway=false` and be ommitted from the configuration file. This makes the simple case even simpler.
Comment out all unused-but-desired-in-the-future settings, remove some
settings that will never be used, and rename `ssdp_discovery` to
`autodiscoverable`.
The add-on code was copy-pasted from other addons and used the GPL v2
license, whereas by accident the LICENSE text file had the GNU "Affero" GPL
license v3 (instead of regular GPL v3).
This is now all streamlined, and all code is licensed as "GPL v3 or later".
Furthermore, the code comments just show a SPDX License Identifier
instead of an entire license block.
Instead of returning an error "error doing X", just return "doing X". The
fact that it's returned as an error object says enough about that it's
an error.
This also makes it easier to chain error messages, without seeing the
word "error" in every part of the chain.
