[Milou] Optimize and cleanup sources model

• Simplify queryString once, rather than during every iteration and use QStringRef where possible
• Foreach instead of potentially detaching range-for
• Use reverse iterators (new in Qt 5.6) instead of Java-style iterators
• Remove traces of begin/endInsert/RemoveRows feature - while it would be nice to not reset the model, with all this sorting and merging going on it's pretty difficult to predict where an item will end up which is probably why it never got implemented. It still calculated tons of indices that went unused, so remove this and go a different route in the future

Differential Revision: https://phabricator.kde.org/D2395