Fragile undo/redo for QgsComposerItems

[nyalldawson]

The current implementation of undo/redo for composer is very fragile:

• QgsComposerItems are never deleted (just removed from the scene) (performance issue - deleted items are still hanging around performing various calculations)
• Leads to crashes like http://hub.qgis.org/issues/11371

This should be reworked so that item history is stored using the XML representation of the items.

Lots of (very boring, very tedious) work!