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/CmdShow.cpp

@@ -142,7 +142,6 @@ int CmdShow::execute (std::string& output)
     " debug.tls"
     " default.command"
     " default.due"
-    " default.priority"
     " default.project"
     " defaultheight"
     " defaultwidth"
@@ -200,7 +199,6 @@ int CmdShow::execute (std::string& output)
     " urgency.inherit.coefficient"
     " urgency.due.coefficient"
     " urgency.next.coefficient"
-    " urgency.priority.coefficient"
     " urgency.project.coefficient"
     " urgency.tags.coefficient"
     " urgency.waiting.coefficient"
@@ -367,15 +365,6 @@ int CmdShow::execute (std::string& output)
     out << format (STRING_CMD_SHOW_CONFIG_ERROR, "calendar.holidays", calendarholidays)
         << "\n";
 
-  // Check for bad values in rc.default.priority.
-  std::string defaultPriority = context.config.get ("default.priority");
-  if (defaultPriority != "H" &&
-      defaultPriority != "M" &&
-      defaultPriority != "L" &&
-      defaultPriority != "")
-    out << format (STRING_CMD_SHOW_CONFIG_ERROR, "default.priority", defaultPriority)
-        << "\n";
-
   // Verify installation.  This is mentioned in the documentation as the way
   // to ensure everything is properly installed.