Fix Embedded plotting (Options->Plot Windows->Embedded) in XMaxima
The plot format "openmath" has been a synonym for "xmaxima" since commit
33999182 in November 2009. During the more recent reworkings of the
plotting functions (in December 2013, appearing in Maxima v5.32), this
synonym seems to have been silently dropped even though the documentation
still states that it should work.
This commit simply updates XMaxima to use "xmaxima" instead of "openmath"
for the Embedded plotting.