[libkwineffects] Deprecate Outline feature

Summary:
KWin had Outline effect during KDE SC times, which had been removed in
KDE SC 4.11. With that effect, also showOutline and hideOutline signals
had been removed, making it impossible to write effects that show
outline. Yet, Outline enum value had been left.

Even though the Outline feature is useless nowadays, in order to keep API
compatibility, let's just deprecate it.

Reviewers: #kwin, graesslin

Reviewed By: #kwin, graesslin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D14188