[effects/sheet] Decrease the default animation duration

Summary:
For some historical reasons, the sheet effect uses animation duration of
500ms. In comparison to other effects that animate the appearing and the
disappearing of windows, 500ms is too high value for the duration.

As the title says, this change decreases the default animation duration
to 300ms. The new duration is more saner.

The opacity is animated to make the effect look smoother with the
shorter animation duration.

BUG: 400021
FIXED-IN: 5.15.0

Reviewers: KWin, graesslin

Reviewed By: KWin, graesslin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D16359