Clean-up the `ProgressBar` widget

[corranwebster]

This has min and size attributes which are part of the creation signature but not actually used.

This should be fixed (or a new IField-based progress bar widget written).

[corranwebster]

Some progress here in #1234 to make clear the distinction between fields (such as a progress bar) and editable fields (such as a slider), but not implemented yet. However they can be without further backwards incompatible changes, so bumping to next feature release.