Update project drop down to show relevant projects when a customer is selected

A customer has asked:

"One initial comment and this is probably true in other areas, it would be nice that once you have selected the client that the available project list was only applicable to that client. Currently both lists bring down the complete set of customers / projects".

Can we add a toggle-able option in settings (enabled by default) and relevant system modifications so that the project drop down is updated dynamically based on customer selected, e.g. show only projects that are attributed to that entity + general projects?

By making this toggle-able, this can be disabled by anyone who wants to be able to occasionally link invoices to projects belonging to a different customer / supplier.