Recently Venus, Jupiter and Mars have been added to Marble's stars plugin.

This task is about adding Saturn and Mercury.

Both are yellow.

• Research for the correct method to find the planet positions in the astrolib documentation

• load the file QPixmap(MarbleDirs::path("bitmaps/stars/star_*_white.png")) and QPixmap(MarbleDirs::path("bitmaps/stars/star_*_red.png"))
• display it at the right position using the pixmap above
• resize the pixmap according to the brightness ("magnitude") the very same way it's done for the stars.
• allow for enabling and disabling the planets inside the settings dialog.
• Add a label for Mercury and Saturn. 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.