second attempt of Delete and port away from internal ScrollView
This reintroduces the refactor of ScrollablePage which was reverted due to last minute regressions.
Refactor ScrollablePage to port it out of our internal implementation of ScrollView to upstream QQC2.ScrollView
port the other users as well, most notably OverlaySheet and Dialog.
Testing on various applications don't seem to produce any noticeable differences
Edited by Marco Martin