Add a widget that provides a better layout for the toolbar.

Widgets can be put on the left, center and middle sections. Items on
the left are left aligned, items in the center are center aligned and
items on the right are right aligned.
Use the new toolbar widget to make sure the status widget stays centered.
Make sure that non-valid projects are not loaded.

Should be replaced someday with a dialog to locate the datafiles.
Improve the looks of the status widget.