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/CMakeLists.txt

@@ -17,6 +17,7 @@ set (columns_SRCS Column.cpp Column.h
                   ColRecur.cpp ColRecur.h
                   ColStart.cpp ColStart.h
                   ColStatus.cpp ColStatus.h
+                  ColString.cpp ColString.h
                   ColTags.cpp ColTags.h
                   ColUntil.cpp ColUntil.h
                   ColUrgency.cpp ColUrgency.h