seqplatesolve crash if STOP pressed

Siril version: master

The goal is to stop the platesolving process of the whole sequence.

When running, if you press the Stop button:

• if the sequence is of FITS images type, Siril purely crashes
• if the sequence is of SER sequence or FITS sequence type, Siril runs in an endless loop

Apart from this bug, it rises one question:

The default behaviour of the platesolver is with the Solve whole sequence enabled.

• assuming it can lead to significant working times (even with a goooood PC), the user can be ...surprised by this excess time.
• plate solving the whole sequence is not always wanted. Depending on the workflow (photometry, lucky)
• I gess (correct if i'm wrong) it's been made for precision astrometry

Maybe the option could be made desabled by default, and enabled by the user for an intended use. (C.?)