Show widget cursor after restoring last override cursor

Show widget cursor after restoring last override cursor

qwidget_qpa.cpp was setting cursor directly to platform window, so
QWindow didn't actually know what cursor was supposed to be shown.

Since QWindow now supports setting cursor and has override cursor
handling there, set cursor via QWindow instead of platform window.
Also changed QGuiApplication override cursor handling so that
it will query the cursor from QWindow if there is no override cursor.
With these changes, widget's custom cursor will be properly shown
when final override cursor is restored.

Task-number: QTBUG-24674
Change-Id: I2d2bb1027779256f9d6de560b6533f45e205ffe9
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>