Bumping minor version and updating Changes.

In preparation for a 0.58.0 release.
disabled confusing extra label on IncrementingPlugValueWidget
Merge branch 'master' of https://github.com/ImageEngine/gaffer
Fixed save/load of user plugs.

When created via the UI, user plugs weren't getting the Dynamic flags set, so they didn't serialise correctly.

Fixes #174.
Merge branch 'master' of https://github.com/ImageEngine/gaffer
Fixed CompoundDataPlugValueWidget non-overridden labels.

The code for setting the width of the label was assuming the label would be a LabelPlugValueWidget, but for non-labelled plugs it was in fact a StringPlugValueWidget, which requires a different method of setting the width.