install

Performs the installation asynchronously.

The application calls this method from its finish page. Update `progres_data` to report a progress value from `0.0` to `1.0` and a current status message.

Example:

 ReadySet.InstallerProgressData progress_data;

 public async override ReadySet.InstallerStage[] get_stages () {
     return yield get_stages_from_backend ();
 }

 public async override void install (ReadySet.InstallerProgressData progress_data) throws ReadySet.ApplyError {
     progress_data.pulse_step = 0.4;

     example_backend.progress_data = progress_data;
     example_backend.events.connect (handle_events);
     yield example_backend.run ();
 }

 void handle_events (EventData event) {
     if (event.type_ == "stage-passed") {
         progress_data.pass_stage = event.stage;
     } else if (event.type_ == "progress") {
         progress_data.value = event.progress;
     } else {
         progress_data.pulse ();
     }
 }