Render optional coordinate grid for planets and moons

This should add support for optional display of a coordinate grid and axis indicators for Planet objects. Some objects like Moon or Mars have very clearly defined coordinate systems observed for centuries, while others had theirs only recently defined. Whenever available, it should be drawable. Also, the central meridian of visible object disk should be added in the object infostring. This may aid in observation planning.

A challenge is Jupiter with visible systems I and II and our current rotation correction for GRS longitude. The grid systems rotate independently from GRS! (Of course our simple rigid Jupiter model cannot rotate like the real gas ball.) Equatorial zone shall display only System I grid, and polar area only System II grid.