Use better Workspace Behavior KCM title and icon

Summary:
Consistently call the KCM "General Behavior" and use `preferences-desktop` as the icon
{F7132843}

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