[Cuttlefish] Synchronize sliders

Summary:
The two sliders (toolbar and responsive toolbar) now match
eachother's values. The responsive toolbar's slider range has been
adjusted to match that of the toolbar's slider for obvious reasons.

Test Plan:
Before:
Sliders have different ranges, positions do not synchronize.
After:
Sliders have identical ranges, positions do synchronize.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24351