add option for multiple prefix definitions

[CWempe]

It would be nice if you could save more than one prefix definition and change between them.

So you could have one prefix definition for your standard projects. And one (or more) when you experiment with your settings. (see #10 )

A simple solution could be to have two (or more) text fields with radio buttons in front of them to select the current prefix definition.

(just a quick mockup):