Handle multiple directories when searching for the theme

This patch introduces support for the "/foo/bar:/foo/baz" syntax for the
configuration key "themes_dir", and make the search go through thoses
first, then try the default dir, and only then fails if no theme has
been found.

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The $PATH-like syntax is a bit extra, I figured that since I was using lists — which are needed for an Debian-specific patch to also look up theme in a system-wide dir.