Refactoring and various API, code, and doc changes/improvements of Scrollable/ScrollableContainer and samples.

- Refactoring: separated a reusable Scrollable mixin (will be shared with the new List widget).
- API, code and doc changes/improvements of the Scrollable/ScrollableContainer.
- Updated ScrollableContainer tests/samples (now relying on LinearLayout)
- samples/mobileapp/app.html: remloved useless interm. DIVs (this also avoids styling issue with Flexbox)