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/args.cpp

@@ -135,7 +135,6 @@ int main (int argc, const char** argv)
     cli.entity ("attribute",  "mask");
     cli.entity ("attribute",  "modified");
     cli.entity ("attribute",  "parent");
-    cli.entity ("attribute",  "priority");
     cli.entity ("attribute",  "project");
     cli.entity ("attribute",  "recur");
     cli.entity ("attribute",  "scheduled");