Write FITS file on separate thread

Summary:
When capturing images, previously once the capture was complete, images were written
to disk, then read back, then displayed in the fits viewer (if enabled), and finally the
next image could be captured. This delayed the next capture by a few seconds.
With this change, the FITS data is displayed using (a copy of) the INDI blob
sent to indiccd, a memory buffer, and the FITS data is written to disk on a separate thread,
which does not block the next capture nor the display of the indi data.
This was only implemented for FITS data. This change also involved some refactoring
in the affected files.

Reviewers: mutlaqja

Reviewed By: mutlaqja

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D24976