Before this change, it was possible to select and copy obscured text from a text field.

This  changes things so that:
- Obscured text fields don't allow copy or cut.
- If a field is both obscured and read-only, then selection is disabled as well (if you can't modify it, and can't copy it, there's no point in selecting it).

This adds a simple example to CustomMultiChildLayout that lays out colored containers in a cascade.

This reverts commit e78f135f.

* Modularize ReorderableListView auto scrolling logic

* comment

* fix test

* addressing comment