Add autofocus on temperature change

Summary:
the user can provide a maximum temperature delta in the Limit
settings section. Between each exposure, if the difference between the
last focus temperature and the current temperature exceeds the defined
delta, then autofocus is triggered.

Test Plan:

• connect a focuser with a temperature probe
• check the temperature delta checkbox
• perform autofocus
• create and start a sequence of several captures
• put your fingers on the probe for a few seconds
• double check that autofocus is triggered on next exposure

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D29821