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.

test/default.t

@@ -39,7 +39,7 @@ if (open my $fh, '>', 'default.rc')
   print $fh "data.location=.\n",
             "default.command=list\n",
             "default.project=PROJECT\n",
-            "default.priority=M\n",
+            "uda.priority.default=M\n",
             "default.due=eom\n",
             "dateformat=m/d/Y\n";
   close $fh;