Accessibility: Don't convert double values to ints when adjusting sliders.

In QQmlAccessible::doAction the code was converting the current value of
sliders, spinboxes etc into an int.  As the value is (or can be) a double
this will often not have the right effect.

Change-Id: I1d131551ab87af626a8d030bebd8099173ff015d
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>