Issue 4656: Let concat-markup fold \char calls into surrounding strings

Issue 4656: Let concat-markup fold \char calls into surrounding strings

This results in

    \markup \column \box { AVA
                           \concat { A V A }
                           \concat { A \char #(char->integer #\V) A }
                         }

using the same kerning in all three lines while previously the third
line differed.