Only hide form when exiting until operations complete

- Downloads will cancel & delete incomplete file
- Converting/Cutting has to finish, I don't know the cleanest way to
cancel it yet
Return if directory couldn't be created
Ask if user wants to create download directory first
See description

- Converting & cropping will now show the actual progress, can be
canceled & will be canceled when application is closing
- ContextMenu will now work with multiple selected items