Project manager: Restore possibility of opening multiple dirs at once

The possibility to open multiple directories at once in the project manager was removed in dd7d8218 (reported in #5896 (closed)). This MR restores that functionality.

I believe that the reason for that removal was incorrect behavior when the root item (the current project item) was among the selected items. Checking for the root item was performed against the item that was clicked (curr_item), instead of the current item in loop (item). Hence, whether the "open directory" entry appeared or not depended on which item was clicked. I've changed the check to use item, so this problem should not be present anymore.