Improve drag & drop and selection in Document Structure Docker

this patch include:
The following features for Document Structure Docker:
- Integration of +/- selection buttons
- Draw thumbnails of dragged pages
- Show context menu on dropping for copy/paste pages.
- Improve moving pages.
Also:
- Move KPrSelectionManager and KPrSelectionToggle classe to libs/widgets and rename to:
KoSelectionManager, KoSelectionToggle (these classes provide +/- selection buttons for QAbstracView classes).
- Selection of pages after move/copy on Custom Slide Shows View.

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