Add fcmdr_json_value_to_variant().

Utility function converts a JSON value node to a GVariant.
FCmdrGSettingsBackend: Parse settings into a GKeyFile.
FCmdrSettingsBackend: Add an apply_settings() method.

Subclasses should override this to apply the settings they've parsed
from the JSON data.  Not sure if I care about completion notification
for error handling, etc.  For now it's just a fire-and-forget method.
FCmdrProfile: Add an "apply-settings" signal.

By default this invokes fcmdr_settings_backend_apply_settings() on each
of the profile's settings backend extensions.