* Remove the import of enthought.resource for core functionality in
    resource_manager.py and i_image_resource.py.

* Remove the dependency on enthought.developer AND enthought.io by refactoring
    dock features modules.

* Remove the dock dependence on TraitsBackendWX by removing the direct imports
  of enthought.traits.ui.wx.

* Need to make use of drag and drop features optional in this project's code
    (see dock, grid, sheet, tree, and viewer/tree_viewer) or help make
    enthought.util's drag_and_drop module backend agnostic.  Right now, it has
    a runtime dependency that wx be installed.