Prompt before overwriting files when saving

When saving the game to a file that already exists, prompt for whether
or not to overwrite the file.

Since this was the first instance of a prompt triggered by another
prompt, this also had to do a minor fix to swap the order of completing
the prompt and clearing it, so that we don't submit the prompt and then
immediately clear it.

package.yaml

@@ -30,6 +30,7 @@ dependencies:
 - containers
 - data-default
 - deepseq
+- directory
 - fgl
 - fgl-arbitrary
 - file-embed