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/Context.cpp

@@ -633,7 +633,6 @@ void Context::staticInitialization ()
   CLI::minimumMatchLength   = config.getInteger ("abbreviation.minimum");
 
   Task::defaultProject      = config.get ("default.project");
-  Task::defaultPriority     = config.get ("default.priority");
   Task::defaultDue          = config.get ("default.due");
 
   Task::searchCaseSensitive = Variant::searchCaseSensitive = config.getBoolean ("search.case.sensitive");
@@ -660,7 +659,6 @@ void Context::staticInitialization ()
   for (i = columns.begin (); i != columns.end (); ++i)
     Task::attributes[i->first] = i->second->type ();
 
-  Task::urgencyPriorityCoefficient    = config.getReal ("urgency.priority.coefficient");
   Task::urgencyProjectCoefficient     = config.getReal ("urgency.project.coefficient");
   Task::urgencyActiveCoefficient      = config.getReal ("urgency.active.coefficient");
   Task::urgencyScheduledCoefficient   = config.getReal ("urgency.scheduled.coefficient");