Add QFlatMap::remove_if

Over the past two days, this has come up twice: Unlike with node-based containers, this loop is quadratic with QFlatMap:

for (auto it = m.begin(), end = m.end(); it != end; /* erasing */)
    if (cond(*m))
        it = m.erase(it);
    else
        ++it;

A linear, remove_if-like loop is not possible to implement with existing QFlatMap API, so we'd need a remove_if member.