…ation`

And align all widgets to the top, so that everything still lines up if the custom widgets use additional vertical space.

- Add `ignoreMissingSource` argument to `shuffle()`, defaulting to `true`
  (the original behaviour).
- Add `shuffleWithDefaultSource()` method, specifying a fallback source value
  to be used when a source is missing.

These both forward to the same internal function, whose signature I didn't
consider suitable for the public API, because of the four permutations of
`ignoreMissingSource` and `defaultSource`-null-ness only three make sense.

This brings it into line with ShuffleAttributes and ShufflePrimitiveVariables,
adding a fair bit of functionality over what we had before.

This stuff all worked thanks to `startup/GafferImage/shuffleCompatibility.py`,
but we don't want to be relying on that forever.

Also add a test that we can load a Shuffle node and its settings from a file
saved from Gaffer 1.3.9.0.

Fixes GafferHQ#3976.

This makes the Widget more useful in technical workflows where you might
want string substitutions, expressions or wildcards.

I can't see a reason not to use it, now it's a bit more versatile.