The API level concept is something used by other projects such as
Android and iOS to allow deprecated features to be removed and the
bahavior to be altered without breaking compability with existing
code.
The same will apply to AwesomeWM. The current API level is "4" and
as long as config use this, no deprecation or bahavior change will
be exposed. If the user sets it to an higher value or we release
the next major version and new users start to use the, then current,
default config, they will use the new API level.
The the far future, if ever, we could fork the entire Lua libraries
to support legacy APIs. This would only require to keep the core
API support for those legacy calls. In the meantime, `gears.debug`
will use this to manage the deprecation and some conditional code
will be added as a last resort attempt to preserve behavior
compatibility while moving forward with breaking changes.
ee80fe052f