LP#1717007 Improve egProgressDialog collision handling

LP#1717007 Improve egProgressDialog collision handling

Resolves a race condition where egProgressDialog.open() is called twice
before the first call completes (i.e. the dialog is opened), leaving 2
open dialogs, one of which cannot be closed because its reference is
lost.

Going forward, attempts to open multiple dialogs will always result in
the most recently visible dialog taking preference.  When collisions
occur, a warning is also issued to the console.

Signed-off-by: Bill Erickson <berickxx@gmail.com>