When the build configuration management changed to using
IdeConfigurationProviders rather than doing everything in
IdeConfigurationManager, the default configuration stopped persisting to
the disk (so changes made are only effective during a session). This is
because the configuration was being added by the manager as an
IdeConfiguration rather than an IdeBuildconfigConfiguration, and
IdeBuildconfigConfigurationProvider knows how to read and write
".buildconfig" files.

The most obvious solution, creating the default configuration in the
IdeBuildconfigConfigurationProvider's load function, doesn't work because the
loads are asynchronous and there has to be at least one configuration
when the IdeConfigurationManager finishes initializing (otherwise the
IdeBuildPipeline will fail to initialize).

Instead, the load interface for IdeConfigurationProviders was changed to
an async/finish pair, so the IdeConfigurationManager knows when the
loads finish. At that point, it can check if a configuration was
restored from a .buildconfig file (in which case nothing needs to be
done) or if the default configuration was added by the
IdeConfigurationManager (in which case the buildconfig provider needs to
be informed of it so it can be persisted when changes are made).

https://bugzilla.gnome.org/show_bug.cgi?id=779240