Open Dialog filter registration

I think I commandeered the Sphere editor. :-)

Anyway, plugins can now register filters with the Open File and Import File dialogs.  I also added an error message if the user tries to open a file that no plugin exists for, better than the old behavior of doing nothing at all. :-)  The error message also shows the file extension, so the user has an idea what kind of plugin to look for.

I did remove the wildcard filter from the script editor, since that was causing it to take binary files, which it then proceeded to corrupt if you accidentally hit the Save button.  To compensate, I added additional extensions to it (including git-related files like .md and .gitignore).  I think a hex editor plugin would be better suited to the wildcard type, too bad Scintilla doesn't have a hex edit mode...