The fields expression and replacement are now handled a list of
expressions/replacements, delimited by new lines.
This allow to perform several regexp replacements in one operation.
This change affects only back-end code. A temptative UI change lives
at D107 but doesn't give satisfaction, as to change between a input
box (to get history) and a textarea (to encode seval expressions) is
rather tricky. A proper combobox/textarea control is needed. If the
UI uses React, it would also be easier to implement his, as we have
the state of the widget distinct of the representation.