Test that beforehide is cancelable
ContentDialog: Update API

- CSS class name changes
  - win-contentdialog-body -> win-contentdialog-dialog
  - win-contentdialog-dimcontent -> win-contentdialog-backgroundoverlay
  - win-contentdialog-content is now public
  - win-contentdialog-commands is now public

- Property changes
  - isPrimaryCommandDisabled -> primaryCommandDisabled
  - isSecondaryCommandDisabled -> secondaryCommandDisabled

- Exposed enum WinJS.UI.ContentDialog.DismissalReason
Remove notion of "active" dialog

This was introduced to catch an edge case which could lead to multiple
dialogs. However, we're going to allow multiple dialogs in the future so
there's no point in introducing this extra code.
Remove setting of aria-hidden until we settle on a solution
Allow beforeshow to be canceled
A standalone button should be right-aligned