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/CmdProjects.cpp

@@ -82,7 +82,6 @@ int CmdProjects::execute (std::string& output)
   std::map <std::string, int> unique;
   bool no_project = false;
   std::string project;
-  std::string priority;
   std::vector <Task>::iterator task;
   for (task = filtered.begin (); task != filtered.end (); ++task)
   {