support global edit mode
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
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
Differential Revision: https://phabricator.kde.org/D24265