Only show panel toolbox when desktop toolbox is open

Summary:
This way the panel toolbox isn't always visible, because with D23951 we're basically always in unlocked mode.

Depends on D24239

Test Plan: Open the desktop toolbox and then the panel toolbox.

Reviewers: Plasma, VDG, ngraham, mart

Reviewed By: Plasma, VDG, ngraham, mart

Subscribers: mart, plasma-devel, VDG, Plasma

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24246