ColorPlugValueWidget : Lazy ColorShooser create

[ericmehl]

This improves the performance of creating a ColorPlugValueWidget by deferring the creation of the internal color chooser to when it is first made visible.

For a single ColorPlugValueWidget, the difference is not all that noticeable, but for a node such as aiStandardSurface, the improved creation time of all the widgets in the node editor is subtly faster.

It's also noticeably faster when testing on top of #6003 where we are adding a number of additional widgets and increasing the amount of time needed to populate the node editor when not deferring color chooser creation.

For me, some timings when selecting an aiStandardSurface node :

• Current code : 0.60 seconds
• Deferred creation : 0.35 seconds (41% shorter)
• New ColorChooser without deferred creation : 0.75 seconds (26% longer than current)
• New ColorChooser with deferred creation : 0.35 seconds (53% shorter than without deferred)

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Gaffer project's prevailing coding style and conventions.

[johnhaddon]

Nice one! Thanks Eric!