Dialog

Work in progress to support Dialog by adding a "center" orientation argument to delite/place and delite/popup (and thus to HasDropDown).

Builds on HasDropDown changes from #307.

Still needs:

1. code for repositioning and resizing the dialog when the focused node changes (i.e. when the keyboard is shown/hidden), and when the viewport changes size (phone rotated).   Maybe expand on _repositionAll().
2. add test for auto-sizing (large centered node)
3. create DialogBase class: tab handling (looping from last focusable element to first)
4. modal support (using DialogUnderlay, plus some code from DialogLevelManager

I looked more into the semantic of HasDropDown's autoWidth and forceWidth, and I now agree they are conceptually incompatible with the center mode. But I still think the current way this is dealt with isn't the right way, the code shouldn't raise "cannot read property corner of undefined" in such a case. So I still think either HasDropDown should skip this code when dropDownPosition is "center", or the properties should be reconciliated.