Undo

- Stubbed the undo implementation in TB2::revert (undo as a name is
  already taken), and in the process lose the capability.
- Add detection of <modifications> when running the 'undo' command
  and generate an error.

src/en-US.h

@@ -203,6 +203,7 @@
 #define STRING_CMD_INFO_MODIFICATION "Modification"
 #define STRING_CMD_INFO_TOTAL_ACTIVE "Total active time"
 #define STRING_CMD_UNDO_USAGE        "Reverts the most recent change to a task."
+#define STRING_CMD_UNDO_MODS         "The undo command does not allow further task modification."
 #define STRING_CMD_STATS_USAGE       "Shows task database statistics."
 #define STRING_CMD_STATS_CATEGORY    "Category"
 #define STRING_CMD_STATS_DATA        "Data"