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/commands/CmdEdit.cpp

@@ -206,8 +206,7 @@ std::string CmdEdit::formatTask (Task task, const std::string& dateformat)
          << "# Status:            " << ucFirst (Task::statusToText (task.getStatus ())) << "\n"  // L10N safe ucFirst.
          << "# Mask:              " << task.get ("mask")                                << "\n"
          << "# iMask:             " << task.get ("imask")                               << "\n"
-         << "  Project:           " << task.get ("project")                             << "\n"
-         << "  Priority:          " << task.get ("priority")                            << "\n";
+         << "  Project:           " << task.get ("project")                             << "\n";
 
   std::vector <std::string> tags;
   task.getTags (tags);
@@ -343,25 +342,6 @@ void CmdEdit::parseTask (Task& task, const std::string& after, const std::string
     }
   }
 
-  // priority
-  value = findValue (after, "\n  Priority:");
-  if (task.get ("priority") != value)
-  {
-    if (value != "")
-    {
-      if (context.columns["priority"]->validate (value))
-      {
-        context.footnote (STRING_EDIT_PRIORITY_MOD);
-        task.set ("priority", value);
-      }
-    }
-    else
-    {
-      context.footnote (STRING_EDIT_PRIORITY_DEL);
-      task.remove ("priority");
-    }
-  }
-
   // tags
   value = findValue (after, "\n  Tags:");
   std::vector <std::string> tags;