[Feature] Paste in place in the composer using ctrl+alt+v.

By using "Paste in place" (ctrl+alt+v), instead of having the elements pasted at the mouse position, they will be pasted at the same page-relative position than they were originally on the page hovered by the mouse.

This is a great time saver when working on multiple compositions having a same layout.

Todo 1 :
IMO, it would be nice if this feature was actually visible to the user.
This would need to have an "edit" menu and/or toolbar with the usual copy/paste/delete actions (it would probably make sense to move that to QActions instead of being implemented in keyPressEvent).

Todo 2 :
For now, pasting on a composer that has a different page height causes inaccurate vertical positioning, since the relative-to-paper origin is computed at paste time upon the destination paper size.

Fixing those would need some deeper modification of the copy/paste system and to the GUI, so, as they aren't essential (especially the 2nd), I'd suggest to pull even without them.