add `draggingFile` attribute to event object for drag & drop events
cross-window fix, drag & drop added to test files

- In addEventHandler(), the check to see if the obj argument was valid
didn't work if the object was in an iframe. If obj is in a different
window, it isn't an instance of this window's Object(); it is an
instance of the iframe window's Object(). (I wasn't aware that they were
different until now.)

- Added to the test_events_max files to test drag & drop events, and
created a new file for use in the iframe.
cross-window fix 2

- Learned that, depending on the browser, window.constructor may not be
the same across windows.
Merge branch 'master' into drag-event-attributes
Merge remote-tracking branch 'origin/master' into drag-event-attributes
Merge pull request #1 from wizard04wsu/drag-event-attributes

add `draggingFile` event attribute for drag & drop events

Because the drag data is protected for most drag events (see http://www.w3.org/TR/html5/editing.html#dndevents), we can't determine if a file is being dragged by simply checking the length of event.dataTransfer.files. Checking for "Files" in event.dataTransfer.types isn't reliable either; e.g., "Files" is included even if you're just dragging an image element.

The dragstart and dragend events are not fired when dragging a file (or files) into the browser window (because the source of the drag is essentially unknown). This fact is used to determine if it's a file being dragged.

- add custom `draggingFile` attribute to the event object for drag & drop events. This indicates if it is one or more files being dragged into the browser window.
- change test files to demonstrate this