support entry of simple filespec wildcards for directories in Edit Source List dialog

[sean-e]

#9 added support for population of the window of directories restricted to a simple filespec.
[dir] and [rdir] support file search spec in form: [[.foo]]
for example: [rdir]c:\bar\baz[[.txt]]

The Edit Source List dialog does not currently support this -- the search spec has to be added to the data file manually.
Add support to the Edit Source List dialog for setting an optional filespec