[effects/slide] Disable "Slide docks"

Summary:
D9638 made docks to slide to "fix" the problem when switching to a
virtual desktop that has a window in full screen mode:

As it turns out, people don't like this kind of behaviour. Another
problem with sliding of docks is that pager goes away.

This change disables sliding of docks by default. One can enable sliding
of docks by checking "Slide docks" checkbox in slide effect KCM.

Yet, transition to/from virtual desktop with a window in full screen
mode doesn't look great but that's somewhat acceptable:

(we don't see issues that are present in the video above because the new slide effect elevates docks if sliding of docks is disabled)

Test Plan: Switched between virtual desktops, the default panel didn't slide.

Reviewers: KWin, Plasma, VDG, ngraham, graesslin

Reviewed By: KWin, Plasma, VDG, ngraham, graesslin

Subscribers: ngraham, graesslin, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D13566