Support "X-foo" environments for OnlyShowIn

[Vincent Untz]

Hi,

Here's a patch that changes the menu spec to support "X-foo"
environments for OnlyShowIn keys. It's useful for people writing new
environments :-)

It's pretty straight-forward, so I don't think there's any reason to not
put this in.

This is https://bugs.freedesktop.org/show_bug.cgi?id=11564
(thanks Stanislav for catching this!)

Vincent

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