editor-frame: Add actions to make search-and-replace work

This commit adds actions so the "Replace" and "Replace All" buttons
work, using the GtkSourceView API. It also adds actions to show or hide
the Replace widgets, to show or hide the search options, and to exit the
search, which are all buttons in the search box.

https://bugzilla.gnome.org/show_bug.cgi?id=765635