Add an option in Advanced>Interface to set starting directory.

By default, Picard remembers last directory used and restores it on start,
but this isn't not always desirable, one may prefer to start on the same directory
each time.
This patch allows one to change default behavior and set a starting directory.
Reduce code redundancy and use starting directory for Add folder action
Add action 'Set as starting directory' to file browser context menu
Change the text for selecting the starting directory

It was "Open this directory when starting:" which is not what's
happening: the selected directory would only be visible if the user is
using the file browser, but not if he's only using the "Add folder"
button.
Add the starting directory to NEWS.txt
Regenerate picard.pot