field 'Age' removed. Refs: #63.
The indirect selection of a sequence number via field 'Age' has been removed.
The sequence number of old sessions is now directly selectable
via a list box with existing sequence numbers plus a first entry named 'new'
for adding a new entry to the history.
ShowHistory has been slightly improved for 'new' entries
in order to align the appearance of new entries in the history
with the information in the dialog box.

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