New UI to allow the user to modify the stretch being used.

Summary:
In fitsviewer, the toggle stretch button is removed and replaced with a
stretch bar at the bottom of the window. It is always there and allows
the user to turn on/off stretching, use the automatically generated stretch
or modify the stretch parameters. As before, this is a display-only change
to the image, it does not modify the image itself.

Test Plan:
In kstars, open a new fits file with the auto-stretch option either set
or not, and see if it displays accordingly. Then switch the other way
using the UI and play with the sliders to adjust the stretch. Try another
image also. Go back to the first image, it should remember the parameters
that were last used. Should work similarly with images from capture.
Evaluate the UI.

Reviewers: mutlaqja, lancaster, wreissenberger

Reviewed By: mutlaqja, lancaster

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D25645