Extension remembers previously entered data

[andydandy74]

Steps to reproduce

• Open extension
• Select source and target dir
• Run migration
• Close extension
• Open extension again
• Notice how all text fields have retained their previous values, including the report

Expected behaviour

Extension should dispose of all the data when window is closed

[mjkkirschner]

this is the case because in implementing this I decided to be lazy and use the Extension itself as the dataContext for the window instead of a viewModel which would get garbageCollected when the window closed.

To fix it I think refactoring out the properties to a viewModel would be good, and setting that as the dataContext of the window.