Saving a mission to a file is by default selecting the filename + extension

The problem is that, if the user renames the file, it's renaming the extension of the file as well, causing the file to not be selectable by default when the user clicks the "import mission".

This happened on macOS.

The solution should be for the name to be selected by default without the extension, so if the user renames, only the actual file name is renamed.

Start by locating the save-mission file dialog and reproduce the filename selection behavior on macOS. Done means renaming the suggested filename leaves its extension unselected, so the saved mission remains selectable through the import mission flow.

Written by the indexing model from the issue text.