SaVi satellite constellation visualizer savi-dev

Updating manual to cover new equator and tropics options.
Also mentioning tilt in Simulation constants, which should
have been mentioned with the 1.6 release.
Updating screenshots to suit.

minor code tidyup.

Adding in menu option for tropics flag.

fixed drawing of tropics for spherical/cylindrical projections,
which need -all-maps flag to even see.

Adding sanity-checking to parameters, and forcing to floating point
so problems don't crop up unexpectedly wth later calculations.

Adding in a 'tropics' flag and checkbox in coverage, to show the Tropics of Cancer and Capricorn.
This can be altered in the Simulation Constants dialog.

Not supporting cylindrical/spherical projections yet, because the trig manipulation escapes me.
So it's unprojected only for now.

Noting unicode support prototype.
Likely a dead end.

Forcing equatorial plane on for equatorial systems.

Minor demonstration of how Unicode support might be done.
Looks painful, but is backwards compatible, easy to override.

lower-case minor text fix.