Fix critical bugs with dialog loading/unloading

Custom window manager is still giving me some trouble in certain areas,
typically those that intermix API and VB window handling.  As always,
"more testing" seems to be necessary...
Improve ExifTool shutdown behavior

(Debugging asynchronous code in VB6 is a joy, let me tell you...)

It's unpredictable, but sometimes ExifTool will not exit normally as
part of the PD shutdown process.  Explicitly giving it a bit of time to
respond to its exit instruction seems to solve the issue on my Win 7 PC,
but I'll need to test this on my other machines to be certain the fix is
universal.