nicer "find in file output", using pango markup

* highlights grep matches and also groups results by files.
* msgwin_msg_add() api is unchanged but text is converted to markup
internally.
* added msgwin_msg_add_markup() and msgwin_msg_add_markup_no_validate().
* added utils_utf8_make_valid() and force valid utf8 in the GTK
widgets, fixes some warnings.
* do not process too many grep results at once to give a chance
to the UI to handle some events sometimes.