Add ScrollView.isScrollEnabled property that should cover the majority of the scenarios that users try to address with isUserInteractionEnabled / isEnabled properties. This is a straightforward and consistent solution that does one thing and one thing only -- enable/disable scroll.

We have a known inconsistency in Android / iOS with respect to the isUserInteractionEnabled property and container components (e.g. scrollview, various layouts, etc.)-- in iOS when property is set to false on the container, it disables user interaction recursively on all subviews, while on Android it doesn't. As this is the standard behavior on each OS we decided not to streamline this:

• on Android isUserInteractionEnabled false is basically equivalent to isScrollEnabled false (if we were to streamline the behavior for scrollview to match iOS counterpart, we would have to do the same for all layout containers too so for the time being we are "consistently inconsistent" -- consistent among Android components and inconsistent betwen Android and iOS)
• on iOS isUserInteractionEnabled false disables scroll and also disables user interaction for all subviews.

Also, ScrollView.isEnabled property currently has no effect neither on Android, nor on iOS.

Fixes #6511
Depends on NativeScript/tns-core-modules-widgets#150 (merge after the widgets PR)