"Revert and Restore tools" and "Manage the Composer" and rewording.

Changes for "Revert and Restore tools" and "Manage the Composer"
Added a little bit about data defined fields as a note at the end of common item properties (I think this means that most things Print Composer in the visual change log are now at least mentioned in this file).

Rewording, mostly around : 
* "Don’t export page if frame is empty" which hopefully makes more sense now.
* The bits of the map window that talk about locking the position of the map item/element.

Lastly, I've added the names for the locked and unlock icons, but these don't appear to be showing when I build it locally. Do they need adding?
|locked| https://github.com/qgis/QGIS/blob/master/images/themes/default/locked.png or svg
|unlocked| https://github.com/qgis/QGIS/blob/master/images/themes/default/unlocked.png or svg

I suspect this also includes PR#372 but I'm not absolutely certain.