Button defaults in "Choosing Destination" dialog

[humdinger]

This is r33893.

Not sure if this isn't a general interface_kit issue, I'll explain the usage in Expander.

Besides the usual "Open File" dialog, there are dialogs for choosing a destination folder, like in Expander. Additionally to ("Cancel" and) "Select", which takes the selected folder from the file listing and closes the dialog, there's the button "Select '{current folder}'".

If this button would always be the default button when there's no other folder in the file listing highlighted, a simple RETURN would choose the current location and make keyboard navigation much simpler. If there is another folder highlighted in the file listing, the "Select" button would become the default and a RETURN would still work as expected.