nicer "find in files" output, using pango markup

[martinbonnin]

• 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.

before:

after:

[elextr]

I don't think this is acceptable as it stands because:

1. it only marks matches to simple search patterns, not regexes, but the regex matches are the ones that really need marking since the match text is not exactly the same as the search pattern

2. it combines two things in one, these should be split

a. forcing encoding to UTF-8 for the text being put in the window, something which should be done anyway

b. highlighting the matches

Given that its quite a big patch and it only partly addresses the problem I don't think its worth the effort in its current form.

[b4n]

Agreed with @elextr. I like some of the ideas there, but some splitting and polishing are required before a merge.