Check whether current R session supports X11 or not.
capabilities("X11") which only checks for X11
support on startup , this function checks whether X11 is supported
when it is called. This is done by querying a temporary R session.
TRUE if X11 is supported, otherwise
 R-devel thread 'capabilities("X11"): Force refresh from within R?
(... and minor documentation issue)' on 2015-05-06,
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