Add IndexedStack widget (#1168)

- Add an IndexedStack widget based on #1137 to Masonry and Xilem
- Update the widget gallery example to use IndexedStack as a tab
switcher

The current implementation uses `set_stashed` to skip layout passes on
inactive children without removing them directly (see [#masonry > How to
handle children that should not be
rendered](https://xi.zulipchat.com/#narrow/channel/317477-masonry/topic/How.20to.20handle.20children.20that.20should.20not.20be.20rendered/with/528888212)).

Author: KiranWells