Delivery app widgets

This should be a relative import rather than an absolute. Also, I think the `custom_widgets.py` module should be moved up from the `ui` folder into the parent level. The current convention is to only keep auto generated python files from `.ui` files in the `ui` folder. If you change the import statement to `from ..custom_widgets import DirTreeView` you can move the file up into the parent level, next to `dialog.py`. (For more information about how to do this in designer, check out this other pull request comment: https://github.com/shotgunsoftware/tk-framework-wb/pull/1#issuecomment-51263730 )

Hope it makes sense! :)