Use different colors for each planet in the Sky Calendar tool, which helps a lot.

Along the way, added a static member QVector<QColor> KSPlanetBase::planetColor to hold the color for each planet (it's indexed on the recent KSPlanetBase enum for major planets).
This way we don't have to repeat hard-coded colors all over the codebase.

I also added a member function SkyMapComposite::planet(int n) that retrieves the planet pointer using the same enum. This lets us avoid using findByName() for planets, although it still uses findByName() internally.

Finally, I am not creating temporary planet objects in SkyCalendar::addPlanetEvents() anymore, because the needed SkyObject functions (riseSetTime() and transitTime() are set up to modify the position of a planet as needed, and then restore the original position when they are finished.

Unfortunately, this didn't have any noticeable impact on the time it takes to populate the plot.