Change/extend import functionality to import into existing database

[GoogleCodeExporter]

I have always found the import function strange in that it always creates a new 
database. It became serious when I exported a game to make some corrections 
with a text editor. It create a file, export.pgn, and after I corrected it, it 
created a new database, export.si4.

What I really wanted was to replace the faulty game in the existing database.

I think the best process would be for the existing behaviour to be an option of 
"Create new database," and for "import" to merge into the current database. I 
would like to be able to maintain a single consolidated database of TWIC games, 
and when I am following a tournament where separate PGN files are published for 
each round, to be able to consolidate those into a single database.

Original issue reported on code.google.com by [email protected] on 5 May 2013 at 7:48