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/Task.h

@@ -37,14 +37,12 @@ class Task : public std::map <std::string, std::string>
 {
 public:
   static std::string defaultProject;
-  static std::string defaultPriority;
   static std::string defaultDue;
   static bool searchCaseSensitive;
   static bool regex;
   static std::map <std::string, std::string> attributes;  // name -> type
   static std::map <std::string, float> coefficients;
   static std::map <std::string, std::vector <std::string> > customOrder;
-  static float urgencyPriorityCoefficient;
   static float urgencyProjectCoefficient;
   static float urgencyActiveCoefficient;
   static float urgencyScheduledCoefficient;
@@ -165,7 +163,6 @@ private:
   const std::string decode (const std::string&) const;
 
 public:
-  float urgency_priority () const;
   float urgency_project () const;
   float urgency_active () const;
   float urgency_scheduled () const;