The stars plugin recently got support for displaying the sun and the moon.

During this task the planet Venus should be added.

• Research for the correct method to find the Venus position in the astrolib documentation
• load the file QPixmap(MarbleDirs::path("bitmaps/stars/star_0_white.png"))
• display it at the right position using the pixmap above
• allow for enabling and disabling the planet inside the settings dialog.
• Add a label for Venus. Reuse the string inside the Planet class.
• Make sure the labels are turned on and off via the context menu and settings dialog just like done with the moon and the sun.