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