comment editing improved. Refs: #63.
Improvements as suggested by Robert and Helmut:
- option 'Force New' removed from dialog
- selecting 'Age 0' means 'Force New'.
- 'Age 0' comment shown in history with 'seq nr=####'
- docu updated accordingly

Dialog.MapString used for header line of history.
Open dialog box set to Age 1 upon Externalize.

Signed-off-by: Josef Templ <josef.templ@gmail.com>