Fixing bug: https://bugzilla.gnome.org/show_bug.cgi?id=628617

The problem was that the sidebar was not updated when the new document was loaded.

To fix that, a call to setup_sidebar_from_metadata was added to ev_window_document_changed_cb.
To avoid multiple calls to the sidebar setup on initialization, it was removed from
setup_document_from_metadata (a call to sidebar setup is added to ev_window_open_document because
of this change).

ev_window_sidebar_set_current_page has an extra case that sets the sidebar to thumbnails by default.
This was necessary because if a document that supports links is reloaded with a version not
supporting links, the link page on the sidebar was still in focus (although 'links' was disabled).