ColTypeDate: Renamed 'ColDate' to 'ColTypeDate'

- Renamed files ColDate* to ColTypeDate*.
- Renamed objects ColumnDate* to ColumnTypeDate*.
- This is mainly to make way for ColType{Duration,String,Numeric}, while leaving
  ColString untouched. ColString is used only for ViewText columns, but may soon
  be replaced by ColTypeString. Unknown.

src/columns/ColModified.h

@@ -27,9 +27,9 @@
 #ifndef INCLUDED_COLMODIFIED
 #define INCLUDED_COLMODIFIED
 
-#include <ColDate.h>
+#include <ColTypeDate.h>
 
-class ColumnModified : public ColumnDate
+class ColumnModified : public ColumnTypeDate
 {
 public:
   ColumnModified ();