Code Reorganization

- Changed the ::measure and ::render methods from pure virtual to
  virtual.
- Fixed bug where recur.indicator used the length of the indicator
  instead of the indicator.
- Implemented ColString.{h,cpp} to support generic Views based on
  strings, not tasks.
- Implemented newly virtual Column:: methods.

src/columns/ColRecur.cpp

@@ -89,7 +89,7 @@ void ColumnRecur::render (
     if (task.has ("recur"))
       lines.push_back (
         color.colorize (
-          rightJustify (context.config.get ("recurrence.indicator").length (), width)));
+          rightJustify (context.config.get ("recurrence.indicator"), width)));
   }
 }