Allow dragging and dropping an image file to open it

[sozysozbot]

reproduce the issue

• Drag and drop an image onto the Pietron window

Observed result

• Since it is fundamentally a web browser, the Pietron window transitions to the image itself
• Once this happens, it becomes completely stuck and unusable

Expected result

• Pietron loads the image as a valid Piet program, just as it does so when the image is supplied by File → Open