Support multi-line input for “search and replace”

[elfring]

Two text input fields are provided for the functionality “search and replace”.
I would appreciate if they can occasionally be extended for the input of multi-line data.

[elextr]

Escape sequences can be used in the search regex and in the replacement string so you don't need multiple lines, but the limited size may be an issue.

Upgrading the GTK entry (which is single line only) in the dialog to a multiline widget may be possible, but somebodys got to do it.