BUG: interpretation of \033 escape sequences in task

The global substitution in the AWK highlighting of prioritized and done tasks also affected the task text itself, not just the inserted color definitions.
Factored out the evaluation of the color variables and interpretation of \033 into a highlight() AWK function.
Added test cases which check that \a, \t, \n, \x.. and \0.. escape characters in the task text are listed as-is, without interpretation.

todo.sh

@@ -636,17 +636,22 @@ _list() {
             s/^ /0/;
           ''' \
         | eval ${TODOTXT_SORT_COMMAND}                                        \
-        | awk '''{
-            pos = match($0, /\([A-Z]\)/)
-            if (match($0, /^[0-9]+ x /)) {
-                str = ENVIRON["COLOR_DONE"] $0 ENVIRON["DEFAULT"]
-                gsub(/\\+033/, "\033", str); print str
-            } else if (pos > 0) {
-                clr = ENVIRON["PRI_" substr($0, pos+1, 1)]
-                str = ( clr ? clr : ENVIRON["PRI_X"] ) $0 ENVIRON["DEFAULT"]
-                gsub(/\\+033/, "\033", str); print str
-            } else { print }
-          }'''  \
+        | awk '''
+            function highlight(colorVar,      color) {
+                color = ENVIRON[colorVar]
+                gsub(/\\+033/, "\033", color)
+                return color
+            }
+            {
+                pos = match($0, /\([A-Z]\)/)
+                if (match($0, /^[0-9]+ x /)) {
+                    print highlight("COLOR_DONE") $0 highlight("DEFAULT")
+                } else if (pos > 0) {
+                    clr = highlight("PRI_" substr($0, pos+1, 1))
+                    print ( clr ? clr : highlight("PRI_X") ) $0 highlight("DEFAULT")
+                } else { print }
+            }
+          '''  \
         | sed '''
             s/'${HIDE_PRIORITY_SUBSTITUTION:-^}'//g
             s/'${HIDE_PROJECTS_SUBSTITUTION:-^}'//g