Make containsMouse property private.

Also removes the RangeTab from the Gallery example, since its controls
use containsMouse and are no longer different from the same set of
sliders on the control tab. Adding MouseAreas to each control in
RangeTab to achieve the same animation would clutter the example with
boilerplate code.

Change-Id: I80d6f3bbf1b73229f4d0ae99dd2a65bc083e7099
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>