Use better Workspace Behavior KCM title and icon

Summary:
Consistently call the KCM "General Behavior" and use preferences-desktop as the icon

Test Plan: Open the General Behavior KCM.

Reviewers: Plasma, VDG, filipf, ngraham

Reviewed By: VDG, filipf, ngraham

Subscribers: filipf, kde-doc-english, ngraham, VDG, plasma-devel, Plasma

Tags: Plasma, Documentation

Differential Revision: https://phabricator.kde.org/D22871