Improve ConfigDialog API for adding pages, follow list API patterns

Summary:
The old API had a single method "add" which then by the second parameter
being a nullptr or not decided whether to insert before that page
as given by the second parameter or append at the end.
This confused code readers a little, so splitting into
two distinct methods following std list API conventions
("append(x)" vs. "insert(before, x)") should improve things,
even if at the cost of an additional symbol.

Reviewers: KDevelop, mwolff

Reviewed By: KDevelop, mwolff

Subscribers: mwolff, apol, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D6038