This change has two implications:
1) tree_render() will now be called precisely once for input which consists of
multiple commands (like "focus left; focus right"). Also, the caller of
parse_command() has to call it. This makes us able to fix tickets such as
ticket #608 (where multiple tree_render() calls are noticable).
2) The output of a command is now a JSON array of return values of the
individual subcommands. In the case of "focus left; focus right", this is:
[{"success":true}, {"success":true}]
While this is incompatible with what i3 returned before, the return value of
commands was undocumented and therefore not subject to our API stability.
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