launching help viewer

Under *nix systems, if we can't find the direct location of docview, we
try the 'which docview' command. If we get a positive response, we know
docview is in the system path, and we can go ahead and launch it.

At least this doesn't instantly throw an exception when docview can't
be found on the first attempt.
New SearchMode property to FileListBase class

This allows the developer to filter the file results by Files only,
Directories only, or both (default).
Merge branch 'dibo-searchmode' into develop
version bump for develop branch