Allow mulitple modal dialogs to work together by maintaing a stack of keydownHandlers, only the top-most of which are enabled.
Rename keydownListeners to _keydownListeners and add JSDoc type annotation; port over @njx's escape-handler fix from 3c6bafd55ccce27644f5aec7eae75981e91ec316.
Address review comments by removing the current dialog's handler from the "stack", even if it isn't on top, and also only fiddling with the KeyBindingManager when there is a single open dialog.
Merge pull request #3785 from adobe/iwehrman/multiple-modal-dialogs

Fix keyboard event handling for stacked modal dialogs