When installing an app in g-s while it is offline, g-s was adding it to
the pending installation queue directly. However, if the app is coming
from a local source like a local repository (USB drive, test repo, etc.)
that behavior is not ideal. So it should be up to the plugins whether an
app installation is performed or queued for later.

To achieve that, this patch changes how the plugin-loader behaves by
allowing plugins to check the state of the network themselves themselves
(by introducing a new method gs_plugin_get_available_network), and
checking the state of apps after calling the gs_plugin_app_install
methods are called: if the app's state is QUEUED_FOR_INSTALL, then it
adds the app to the installation queue.

This means that in order to keep supporting this behavior in their
platforms, plugins need to check whether apps should be queued for
installation, set the mentioned state to them, and return TRUE (in
the app install override).
This patch changes that already for the PackageKit and Flatpak plugins.