Allow ++ as an alternative separator to -- for user CLI arguments

[Calinou]

This is required when using startx to start a Godot project, as -- is used by startx for its own arguments (and there is no way to escape it). This is specifically needed by https://github.com/godotengine/godot-benchmarks for automation purposes.

Testing project: test_cli_separator_double_slash.zip

[akien-mga]

// looks pretty awkward to me, especially on Unix command line where it is a valid path ls //home works like ls /home.

Maybe we could use --- instead?

Edit: Though it's not too terrible either, if others are fine with // I don't mind.
But we could maybe pick a character which doesn't have an existing meaning in usual shell/bash. See "special characters" table: https://www.oreilly.com/library/view/learning-the-bash/1565923472/ch01s09.html
E.g. ++ (opposite of --) or %%, @@.

What's the startx command you need to run exactly? Is there really no way to separate startx arguments from the launch command's arguments?

Edit: Did some tests myself, couldn't find a way aside from making a shell script.

[DarkKilauea]

Yea, I'd rather see something other than //, since it is a valid path or looks like a comment in C derived languages.

--- seems like a reasonable workaround. The other options suggested by akien-mga seem fine too.

[Calinou]

--- seems like a reasonable workaround. The other options suggested by akien-mga seem fine too.

--- might be intercepted by some apps (not startx specifically), and it might look like a typo at a glance.

I've changed this PR to use ++ instead of //.

[akien-mga]

Thanks!