When the map theme indicates that its target is the "sky" then the graticule plugin should draw declination and right ascension lines.

The declination lines can just be treated like latitude lines.

The right ascension lines however range on an clock scale:

While the longitude scale goes from -180 deg to 180 deg, the right ascension scale goes from 0 - 23 h  (or 24 h for a full turn).

Every hour is split into 60 mins (0-59min) and every minute into 60secs (0-60secs)

The grid should be drawn accordingly.

For labels the units would then be "h", " ' ",  " " ": e.g. 15h 24' 12".

In order to create proper strings a new Notation enum has to be introduced inside GeoDataCoordinates called "Astro" this enum should not be visible in the settingsdialog.

GeoDataCoordinates::toString() and GeoDataCoordinates::lonToString should receive a simple implementation that ignores the precision.

For organisational reasons this task can only be started after the UTM feature is implemented