We shouldn't use the /tmp folder directly because if the app is flatpaked the /tmp folder is not accessible by the host.
This patch changes the default DL file location to the Download folder, so it's not possible to loose the work when the user closes the app without changing the file location. By default downloaded po files will be stored on $HOME/Downloads.
With this new behavior we don't need to ask to the user for a new location just after the file is downloaded. We just stored the file in Downloads by default and the user can do a normal app usage.
Fix #117 (closed)