Create the "Choose primer library" setting in the "Primer library" window

Problem:

"Primer library" is a good feature, but it's not convenient to work with if there are too many primers. 

Solution:

We already have the setting "File storage" - this is the directory for the "Primer library" database (and several other things). From now, the place where primer libraries are saved shouldn't be dependent on this directory. Instead, the new button "Choose primer library" has to appear in the "Primer library" tab. When you click on it, the dialog with the list of libraries appears and you may choose one.