Ability to add custom open dialog for split editor area empty widget.

Applications need ability to open custom open dialogs (easier than
writing the entire empty_widget), since the editor area does not
know the directory to default to and the file filters to attach
to the dialog.
Merge pull request #82 from pankajp/tasks-split-editor-area-empty-widget-custom-open-dlg

Ability to add custom open dialog for split editor area empty widget.