proof-shell-trace-output-regexp in trace output

[Makarius]

proof-shell-trace-output-regexp appears visually in the *trace* buffer.

For example (in Isabelle):

ML_command {* tracing "foo" *}

Results in

\^AVfoo

Another example with extra term markup, which is displayed properly after the initial garbage:

ML_command {* tracing (Syntax.string_of_term_global @{theory} @{term "x == y"}) *}

[Makarius]

Works after adding special "V" to invisible parts of isar-output-font-lock-keywords-1.