Surface input fetching cancel button

[masonwillman]

We need to implement a cancel button on the progress dialog for surface input downloads. The Qt4 GUI would allow the download to finish, delete the downloaded file, and then allow the user to start another download.

For the Qt6 GUI, we should have a signal cascade from the GUI to the API that then stops the fetch loop. This will need implemented for SRTM, GMTED, and LCP fetching.