Stars plug-in (Moon and Sun) rendering improvements

* Sun pixmap updated
* The size of the sun is rendered the same way as we render the moon right now including diameter calculation and magnifying mode
* The backdrop-glow is rendered separately before rendering the sun
* The moon is rendered after the sun has been rendered (so that it can eclipse the sun during eclipses)
* Labels added
* The functionality provided is synced with the settings dialog.

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