XEP-0392: theme, ui: conditionally colorize user nicks

* add theme_hash_attrs()
* when printing a user message check PREF_COLOR_NICK to decide whether
  to colorize it or not

ideally we should hash the jid instead of the nick but this is already
a first step.

src/ui/window.c

@@ -1453,6 +1453,10 @@ _win_print(ProfWin *window, const char show_char, int pad_indent, GDateTime *tim
             colour = theme_attrs(THEME_THEM);
         }
 
+        if (prefs_get_boolean(PREF_COLOR_NICK)) {
+            colour = theme_hash_attrs(from);
+        }
+
         if (flags & NO_COLOUR_FROM) {
             colour = 0;
         }