Use EvPageActionWidget directly in the toolbar instead of EvPageAction

We will get rid of current EvPageAction when the previewer doesn't use
GtkUIManager for the toolbar.
Make EvPageActionWidget work when the document model already has a document when it's set
ev-zoom-action: Use g_signal_connect_object to connect to EvDocumentModel changes

To make sure the signals are not emitted once the zoom control has been
destroyed.
shell: Use a GtkHeaderBar instead of a GtkToolbar and window decorations

Based on patch by Yosef Or Boczko.

https://bugzilla.gnome.org/show_bug.cgi?id=711078