Randomly discovered during some other bug hunt, only with
AddressSanitizer on. Basically, ASAN detected stack-use-after-return
when playing with spinboxes in the Light Sources preferences when Qt
signal handlers tried to invoke lambda callbacks that referenced
deallocated stack memory.
The main problem is that those lambda functions in the constructor
were captruing by reference. When they were connected to Qt signals
and invoked after constructor completed, they accessed stack vars that
have gone out of scope.
So fixed that by changing lambda captures from `[&]` to explicit captures:
- [this] for lambdas needing only class member access
- [this, updateLight] for lambdas that need both class members and the
`updateLight` lambda.
ce1ca376f1