This patch should allow users to optionally use sextractor for source extraction for plate solving instead of relying on python.

This will enable plate solving without python3 on OS X, and possibly faster plate solving on linux.

I also added some tooltips where I found that they were missing.

Try plate solving with and without "use sextractor not python" selected on Linux and OS X.
Even better, try plate solving with a messed up python installation using sextractor.
It should still work.