Priority: Converted 'priority' attribute to a UDA

- 'priority.long' and 'priority.short' formats now map to 'priority', and
  generate a warning.
- Man page updated.
- Converted priority urgency coefficients to UDA equivalents.
- Converted priority color rules to UDA color rules.
- Removed 'default.priority' support.
- Removed special sort handling for 'priority' field in Variant, added special
  UDA sort handling.
- Removed ColPriority.{h,cpp} source files.
- Removed asorted newly-unused variables.
- The 'show' command now highlights unused priority settings.
- Removed unused localized priority-relateѕ strings.
- Added legacy mapping for columns and sort columns for 'priority.long' and
  'priority.short' columns in report definitions.
- Removed priority color rules implementation.
- Removed obsolete tests for #860, #990, custom.priority_long.t.
- Updated various tests that set priority default, colors.

src/sort.cpp

@@ -79,8 +79,6 @@ static bool sort_compare (int left, int right)
   int right_number;
   float left_real;
   float right_real;
-  char left_char;
-  char right_char;
 
   std::vector <std::string>::iterator k;
   for (k = global_keys.begin (); k != global_keys.end (); ++k)
@@ -133,25 +131,6 @@ static bool sort_compare (int left, int right)
                        : (left_string > right_string);
     }
 
-    // Priority.
-    else if (field == "priority")
-    {
-      left_char  = ((*global_data)[left].get  (field))[0];
-      right_char = ((*global_data)[right].get (field))[0];
-
-      if (left_char == right_char)
-        continue;
-
-      if (ascending)
-        return (left_char == '\0'  && right_char != '\0')                           ||
-               (left_char == 'L' && (right_char == 'M' || right_char == 'H')) ||
-               (left_char == 'M' && right_char == 'H');
-
-      return (left_char != '\0'  && right_char == '\0')                           ||
-             (left_char == 'M' && right_char == 'L')                          ||
-             (left_char == 'H' && (right_char == 'M' || right_char == 'L'));
-    }
-
     // Due Date.
     else if (field == "due")
     {