Remove the redundant kis_slider_spinbox headers
Removed the kis_slider_spin_box.h from inside libs/widgetutils to libs/ui/widgets and also the kisdialogstate saver along with the tests.
Moved the KisDialogStateSaver too since I couldn't think of anything other at this moment, if there is a better approach to deal with that, please do let me know.
Krita should build and work like always, there should be no changes.
- I confirmed this builds.
- I confirmed Krita ran and the relevant functions work.
- I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
- I made sure my commits build individually and have good descriptions as per KDE guidelines.
- I made sure my code conforms to the standards set in the HACKING file.
- I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.