Summary:
When using multiple screen with a Folder View as a desktop containment, we need to make sure a file belongs only to one screen. The patch adds support for this by:

1) Introducing a ScreenMapper object shared acrossed all Folder Views
2) FolderModel registers the screen it resides on if used as a containment
3) FolderModel filters out items not on the current screen (if used as a containment)
4) FolderModel adds mapping for the newly appeared files. The new files will go to the registered screen having the smallest id. This also means by default all files appear on the screen with the smallest id.
5) url/screen mapping is stored in the configuration of each folder view applet. This means duplication of the information, and although they should be in sync unless manually modified, the logic is that the last view's mapping option is used.
6) When removing a screen, the list of items from the removed screen is stored and items get moved to the first screen
7) When a screen is added back, the items are moved back there based on the above list

So far missing:
- correct handling of D&D between folderviews on different screens
- forgetting the saved position from a removed screen if the items are reorganized on the primary (visible) screen

Depends on D8566
Depends on D8567

Reviewers: #plasma, ervin, mlaurent, dvratil, hein, aacid, apol, mwolff

Reviewed By: #plasma, hein, mwolff

Subscribers: anthonyfieroni, ngraham, mwolff, davidedmundson, broulik, mart, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8493