Related to #7433
This extracts our "launched from desktop" logic into a config option.
The default value is detection using the same logic as before, but now
this can be overridden by the user.
This also adds the systemd and dbus activation sources from #7433.
There are a number of reasons why we decided to do this:
1. It automatically gets us caching since the configuration is only
loaded once (per reload, a rare occurrence).
2. It allows us to override the logic when testing. Previously, we
had to do more complex environment faking to get the same
behavior.
3. It forces exhaustive switches in any desktop handling code, which
will make it easier to ensure valid behaviors if we introduce new
launch sources (as we are in #7433).
4. It lowers code complexity since callsites don't need to have N
`launchedFromX()` checks and can use a single value.
