embed: Not every toolbar has a searchbar

For example, EditToolbar does not have one.

Fallout from 13954c74f593806a5111bea2129bbaa28423fab0

https://bugzilla.gnome.org/show_bug.cgi?id=719920
Avoid resetting the titlebar on a realized window

https://bugzilla.gnome.org/show_bug.cgi?id=724306
selections: Handle the sharing dialog being deleted

https://bugzilla.gnome.org/show_bug.cgi?id=726343
places: Use the builtin GtkHeaderBar in GtkDialog instead of our own

https://bugzilla.gnome.org/show_bug.cgi?id=726343