editor-frame: Add search-and-replace to the search UI

This commit implements Allan's designs for the search-and-replace UI,
which add a replace text entry field, Replace and Replace All buttons,
and search option check boxes to the existing search UI. Project-wide
replace has not been implemented yet, and the "Search selection" option
was not included because the current selection is the default search text.

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