[giac] improvements for nroot(), (-8)^(2/3), real(), imaginary(), conjugate()

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@24763 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
Make sure powers are printed correctly in XML and autotest

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@24764 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e