Fix performance bottleneck in QQuickWindow::mouseMoveEvent().

Fix performance bottleneck in QQuickWindow::mouseMoveEvent().

When a mouse move event is received, a lot of time is spent looking
for items with cursors, recursively.

With this patch, it will only recur into item hierarchies that
contain cursors.

Not having cursors is much more common than having them.

Benchmark included:
Before: 15 msecs per iteration (total: 62, iterations: 4)
After: 0.000064 msecs per iteration (total: 68, iterations: 1048576)

Task-number: QTBUG-27054
Change-Id: I3a5441652ca1c0b8d2cbc5683013562174f5af4a
Reviewed-by: Alan Alpert <416365416c@gmail.com>