Polar Alignment assistant: Platesolving Polar Alignment

[GuLinux]

This implementation will be similar to PoleMaster & co.

While pointing around the pole, it will take 2/3 images in sequence, rotating (manually or automatically) the mount in RA of some degrees (60-90).
It will plate solve at each step, and calculate the drift angle in azimuth and altitude.

It will then assist the user in aligning to the pole, taking new shots and suggesting what movements to take at each step (still plate solving, probably).
Parent issue: #31