[TIMOB-9113] iOS: Having a property to control opacity for Ti.UI.ScrollableView page dots

*Summarized description* There should be a property to change the visibility of the Ti.UI.ScrollableView paging dots, something like 'pagingDotOpacity'. See screenshot of more info. *Original description* Since SDK 1.7 I've had to implement a small (but reoccurring in each SDK update) fix as outlined by fellow Titan; Adriano Paladini, for making the pagingControl semi transparent (http://developer.appcelerator.com/question/42021/scrollableview---manually-position-the-pagingcontrol). The link above shows code which has worked in each SDK I've updated too, would be great to have this implemented as part as the API in future releases.