Realistic meteor showers

At the moment, Stellarium can show meteors, but they are simply decorative - they appear at random points at a rate set by the user. The existing code of the MeteorMgr class can be used as a base for a plug-in that shows more or less scientifically accurate meteor showers. They are not random - the meteors appear to "stream" from a single point in the celestial sphere, the radiant.

(See the Wikipedia article on meteor showers for more information. A lot of time-lapse photographies of meteor showers that show the effect can be found on the Web.)

This feature can take several forms:
- Weak form: Allows the user to define a meteor shower and/or pick it from a list, and then to turn it on and off on a whim.

- Strong form: Keeps a meteor shower catalogue as with the other kinds of objects tracked by Stellarium, and shows only what should be in the sky for the given date. The catalogue should contain information about the radiant and the annual changes in the zenith hourly rate (as meteor showers typically have a peak and are active some time before and after that; a normal distribution function can do).

- Very strong form: Use a professional model for predicting meteor showers, based on the orbits of the clouds of space particles that cause them.