Emit loadProgress only between loadStarted and loadFinished

We now track the values we send to the loadProgress signal. We store
the last value in a new int WebContentsDelegateQt::m_lastLoadProgress
and only send values that are >= m_lastLoadProgress to ensure
monotonicity.
A value < 0 indicates that no loading is going on. Only on loadStarted
it is set to 0 (and a loadProgress is emitted with progress 0) and
on loadFinished, it is set to -1 (and a loadProgress is emitted with
progress 100).
This way, we ensure that you first get a loadStarted signal, at least
two loadProgress signals (0 and 100) and a loadFinished signal
AND all loadProgress signals come in a monotonous order.

Task-number: QTBUG-57839
Task-number: QTBUG-61815
Change-Id: I219d3bffbd5691adb892a11b79647ba9e1ed248e
Reviewed-by: Michal Klocek <michal.klocek@qt.io>