Rename "Desktop Behavior" category to "Workspace Behavior"

Summary: "Desktop" to users means the desktop folder, so use a more appropriate name for this category.

Test Plan: Open the system settings.

Reviewers: Plasma, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: ngraham, VDG, plasma-devel, Plasma

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D22872