Rotate imported background

@sebojolais: Just a brain dump of things which come to my mind when thinking about manipulating the background:

User experience

What are currently the ways to set an object as background?
- Import in document mode
- Context menu of item delegate in Board mode
- Long-press "Add page" and import a PDF or image
- Drag-and-drop or copy/paste a PDF document to the board -> pages are appended at the end of the document
What is the current way to unset the background object.
- Long-press on "Erase" and then erase background.
- But there is no way to detach it without erasing.
When rotating an image and then setting it as background object, the rotation is lost.
What about some means to detach the background object, rotate it and attach it again?
There might also be use cases for a rotation of the background which is not a multiple of 90°, e.g. a slightly rotated scanned page.
The UI element (button, menu entry or whatever) for detaching or rotating the background should only appear on pages which have a background set.
What about attaching a delegate (the gray frame) to the background? Then we have something where we can add buttons, a menu, manipulation handles etc. But how do we select the background? Definitely not with every click on an empty place of the scene!
A PDF page is currently the only object which can only exist as page background. Why? Would it not also be a use case to handle it like an image or any other object?

The XPDFRenderer already has a rotate parameter, see line 208. But this is most probably only in 90° steps.
The background object already has a transform attribute, but it seems to be only used for translation. However if I modify that manually, then I see that this parameter is observed when rendering the PDF. So it could be used to rotate the background PDF by an arbitrary angle.
There is some code in downloadFinished which even can set a widget as background object, but I think this is nonsense and just a leftover from long time ago.