feat(kcm): control scale

Summary:
Some backends might not transmit scale information. For these save the scale
value in the control file on change.

Loading it from there is unproblematic in all three cases:

• No scale value in file. Then falls back to libkscreen's value (default = 1).
• Scale values are the same, that means backend transmits scale.
• Backend does not transmit and scale is different, then prefer control file.

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D26481