DB Editor Export functionality could append to existing DB instead of only overwrite

[DillonJ]

The export functionality in DB editor is really useful - I mostly use it to move stuff from one DB to another. Currently, you can export a selection of objects to a sqlite DB - however if the DB already exists the only option you have is to overwrite it. It would be really useful and possibly very easy to implement to add the option to append/overwrite the data in the destination DB, rather than overwrite the entire DB.