preview: The toolbar is created after load-{started, finished}

Fallout from cde99212000c2f2b0beb29d66956aad7b791b44e

https://bugzilla.gnome.org/show_bug.cgi?id=700716
documents: Let DocCommon children indicate whether they support sharing

Create a new canShare virtual method, which can be implemented by the
individual classes. This is better than littering the rest of the code
with source-specific code.

https://bugzilla.gnome.org/show_bug.cgi?id=725128
documents: Warn that DocCommon implementations must override canTrash

https://bugzilla.gnome.org/show_bug.cgi?id=725128