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

This method (popup.open()) used to always return an object. This is no longer the case, while HasDropDown still does:
```
var retVal = popup.open(...);
...
if (retVal.corner[1] === "R") { ... }
```
(around https://github.com/wkeese/delite/blob/dialog/HasDropDown.js#L484) thus raising an error when accessing the property of retVal. 
It's not just a matter of dong `return this._positionAndSize(args);`. Additionally, popup.center() must also return an object, just as popup.around()/at().