We accidentally did show the command as a list in to_str(). However, after
correcting that to use shlex.escape, we got ugly qutebrowser command lines
when tabbing to the default value, because of how shlex handles double-escaping:
>>> print(shlex.quote("gvim -f '{}'"))
'gvim -f '"'"'{}'"'"''
While in this case, outputting "gvim -f '{}'" would be much more appropriate, it
doesn't look like we can teach shlex.quote to do that.
Instead, we now only accept a list as input for ShellCommand, at the price that
the user needs to do
:set editor.command '["gvim", "-f", "{}"]'
instead of
:set editor.command 'gvim -f {}'
Fixes#2962.
