Summary:
desktop and panel toolboxes are only visible in "edit mode"
edit mode can be triggered by
* long press in desktop
* context menu action
* dbus property (to be eventually used by systemsettings)
The desktop toolbox becomes a "toolbar" with just few selected actions
the presence of that toolbar both fullfills "long press should usually show some
contextual actions" and being a very visible hint that "we are now in edit mode"
toolbox is still movable around in case would obscure any important stuff where it's
by default

Depends on D24263
Depends on D24264
Closes T10402

Test Plan: ui works as planned, toolbox position is saved and restored correctly

Reviewers: #plasma, ngraham

Reviewed By: ngraham

Subscribers: davidedmundson, ngraham, GB_2, plasma-devel

Tags: #plasma

Maniphest Tasks: T10190, T11094, T10402, T8751

Differential Revision: https://phabricator.kde.org/D24265