Q-Range slider fix for linear fits

[krzywon]

This fixes the issue noted in slack regarding an error when running a linear fit. This same error was also thrown when closing any plot with q-range sliders.

The signal used to link the GUI input to the sliders was changed from textChanged to editingFinished, but the disconnection was not changed to match.

[butlerpd]

humm... probably that slackard @butlerpd approving without really testing properly. I'll have a word with him about disciplined and professional 😆

[rozyczko]

This fixes the issue both for linear fits and for closing the charts.
The code is OK.

[wpotrzebowski]

Thanks @krzywon! I will test on Mac

[wpotrzebowski]

ready for testing on OSX

[wpotrzebowski]

ready for testing on Win

[wpotrzebowski]

ready for testing on OSX

[wpotrzebowski]

The qrange bug is indeed fixed. I am not sure if range on the Linear Scale works correctly though. First of all it doesn't seem to be editable (or I don't know how to activate it) and secondly as soon as I start changing values in the log range, linear range gives strange values (outside q range).

.

[krzywon]

@wpotrzebowski, a lot of what you're seeing is documented in #1415, but please add more there if what you're seeing is different. I don't plan to go beyond fixing the error thrown in this PR.

[rozyczko]

Can this be merged, please?

[wpotrzebowski]

ready for testing on OSX