Normally the backend is made to have a singleton instance, and handle all profiles.
However, it may be useful to run instances independently (e.g., for development, to run
components in independent processes, for testing, etc).
This was previously possible by using a specific configuration files at backend's root.
This patch introduce a cleaner way by using an isolation mode.
Isolation mode is activated by settings the `LIBERVIA_ISOLATION` environment variable to
the name of the isolated instance. `LIBERVIA_ISOLATION_MODE` can be used to specify one of
the following mode:
- normal (default): config and shared files are in XDG in a `isolated/<name>` subdirectory
(usually `~/.local/share/libervia/isolated/<name>` and
`~/.config/libervia/isolated/<name>`). Runtime files and other instance specific data
(such as D-Bus name) are also isolated. Config uses cascade as usual.
- full: same as above, but config isn't cascading, it's using only its own isolated config
file.
- tmp: same as normal but the local directory is put in a temporary directory, and deleted
on backend startup. It stays on termination, so data can be checked if necessary (this mode is
usually used for development).
This feature will notably be useful to test severless mode on a single machine.
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