deliteful/ScrollableContainer

[AdrianVasiliu]

Contains:

• A container widget with scrolling capabilities (using delite/Scrollable).
• Intern unit tests.
• Functional tests.

Depends on the following pull request:

• delite/Scrollable mixin delite#79 (delite/Scrollable mixin)

The functional tests (and only them) additionally depend on:

• Add DisplayContainer/DisplayController (display/transition machinery for dapp) delite#75 (DisplayContainer/DisplayController etc.)
• Add ViewStack, LinearLayout, SidePane and ScrollableContainer #7 (ViewStack/LinearLayout etc.)
  (these dependencies are due to the use in these tests of deliteful/LinearLayout).

[AdrianVasiliu]

This PR is now superseeded by #20.
Besides updates to cope with the test packaging changes that occurred since my initial PR, it also addresses both concerns in Bill's comments (inheritance from Widget and reuse of the shared test cases from delite).