SaVi satellite constellation visualizer savi-dev

Updating dialogs to match minor changes in SaVi 1.6.

We lose the titlebars as these vary between systems.
At some point all images will need updating together.

Removed warning about using 1.9 or later, since we'll be on 1.9.5
forevermore.

Finally mentioning 1.6 in COPYRIGHT.

Updating TLE dialog, now that we say Download
and give a shout out to Celestrak.

Updating constants screenshot to reflect updated
constants dialog, with easier-to-understand
rotation period.

We're now into development for 1.6.1.

Minor documentation spacing.

Download is clearer terminology.

saying "Download" is better.

Updating zlib version example.