Add exit on timeout flag to desktop launcher file

[confluence]

The XDG / freedesktop.org desktop launcher used on Linux spawns a terminal window running the backend, which in turn spawns a new browser tab / window. When the browser tab is closed, the terminal remains open (because the backend is still running). So if the user launches CARTA again they'll get a new backend.

Adding --exit_timeout 0 flag to the launcher executable causes the backend to exit when the browser tab is closed, which causes the terminal to close, which seems like more sensible default behaviour.