Add Plasmoid that shows multiple other plasmoids in a tabbed interface

Summary:
Restore the "grouped" plasmoid of KDE4.

It's somewhat similar to the system tray internally, it creates a
containment then has a thin wrapper to display plasmoids in a tab.

Plasmoids will display their compact/full representation depending on
the size of the tab, if compacted, the full representation will appear
as a dialog just outside the tab area.

Test Plan:
Dragged some applets in
Right click, removed an applet
Tested restoration after shutdown
Tested applets with popups (like analog clock or kickoff)
Tested remove/restore

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3620