Initial commit of drag and drop using features and declarative functions.
Add types for drag drata
Remove unused event filter.
Call widget's mouseMoveEvent after custom mouseMoveEvent.
Fix order of drag_data default values.
Make Drag and Drop feature flags ORable.
Change drag data type from bytearray to str.
Add docstring to drag and drop example.
Pass DragEvent's to drag and drop functions so that data can be lazily fetched.
Merge pull request #161 from brett-patterson/drag-drop

Drag and drop
Add an abstract mime data class
add a Qt implementation of MimeData
define a mime data factory method on the application
implement the mime data factory for Qt
define the abstract drag event class
implement the Qt concrete DragEvent class
update docs
make drag-drop api more sophisticated
update the Widget drag drop api
update argument name