Bugfix for PHD2 guiding - restart guiding after lost star

Restarting guiding after a lost star event can lead to the situation that Guide switches to the calibration state while the PHD2 remains in the stopped stage. If in this case guiding needs to be stopped again (e.g. there is no guiding star visible), PHD2 does not send an abort event and Guide remains in the calibration state.

This is fixed by this change. Additionally, a dedicated dithering state is introduced for PHD2 to separate calibration from dithering.