Use colorful moon-style icon for astronomical events plugin

`globe` is not semantically correct because it depicts the Earth, while
astronomical events are de facto about things happening not on the
earth.

plasmacalendarplugins/astronomical/astronomicaleventsplugin.json

 {
     "Name": "Astronomical Events",
     "Description": "Calendar plugin for displaying astronomical events",
-    "Icon": "globe",
+    "Icon": "weather-clear-night",
     "ConfigUi": "astronomicalevents/AstronomicalEventsConfig.qml",
     "Authors": {
         "Name": "Friedrich W. H. Kossebau",