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/CmdInfo.cpp

@@ -150,14 +150,6 @@ int CmdInfo::execute (std::string& output)
       view.set (row, 1, task->get ("project"));
     }
 
-    // priority
-    if (task->has ("priority"))
-    {
-      row = view.addRow ();
-      view.set (row, 0, STRING_COLUMN_LABEL_PRIORITY);
-      view.set (row, 1, task->get ("priority"));
-    }
-
     // dependencies: blocked
     {
       std::vector <Task> blocked;
@@ -420,7 +412,6 @@ int CmdInfo::execute (std::string& output)
     urgencyDetails.add (Column::factory ("string", "")); // Result
 
     urgencyTerm (urgencyDetails, "project",     task->urgency_project (),     Task::urgencyProjectCoefficient);
-    urgencyTerm (urgencyDetails, "priority",    task->urgency_priority (),    Task::urgencyPriorityCoefficient);
     urgencyTerm (urgencyDetails, "active",      task->urgency_active (),      Task::urgencyActiveCoefficient);
     urgencyTerm (urgencyDetails, "scheduled",   task->urgency_scheduled (),   Task::urgencyScheduledCoefficient);
     urgencyTerm (urgencyDetails, "waiting",     task->urgency_waiting (),     Task::urgencyWaitingCoefficient);