Open directory as project by default
When Builder is passed a directory as a parameter (e.g. via
gnome-builder <dir>, or via the "Open With Other Application" option for the directory in GNOME Files), it currently starts in editor mode, and shows the contents in an inline single-file picker (i.e. the same as
gnome-builder -e <dir>).
In either case (terminal/GNOME Files), this is not a useful default, since the user can just as easily select/pass the file they want to open if they want editor mode.
Instead, Builder should open a directory as a project if it's passed as a parameter with neither
-p, since this is useful when the directory is opened via e.g. GNOME Files, is likely what the user wanted if they're passing a directory.