Plasmashell : add dialog to manage containments when in edit mode

Proposal for a UI to manage containments, the main use case is to recover lost or inactive panels from a display that you can no longer access.

qml part: plasma-desktop!618 (merged)

related to https://phabricator.kde.org/T14346

CCBUG: 447044

Access the dialog from Toolbar in Edit Mode (Manage containments)

To show the dialog, simply enter in edit mode, then you can :

• See known displays and their panels and desktops, the dialog automatically update when adding/moving/removing a panel.
• Delete a panel or a desktop
• Recover a panel by moving it to an active display (click on move icon, then select the proper action in drop menu)
• Recover a desktop by moving it to an active display, in fact, this will swap it with th targeted desktop
• Modifications are automatically applied

What I've tested:

• Adding a panel, adding some applet on it. Then disabling its display, open the dialog and move it to an active display, restarting session, the panel is here with its applet.
• On another display, add applets (sticky note) on the desktop. Disable this display, open the dialog and move it to an active display, restart session, both desktop were swapped and you see the background (if different) and sticky note available.

TODO

• Move dialog activation to the edit mode menu bar
• Retrieve the right desktop background image (currently hardcoded)
• Handle more than one panel at the same position
• Rework the dialog's look and feel