StandardNodeGadget : Persistent nodule labels

[ericmehl]

This adds the ability to add metadata to a node to either always show or always hide the node's nodule labels. If there's no metadata entry, the current behavior prevails and the labels are shown on hover.

I'm handling input and output plugs independently here - it seems like there could be cases where you'd want the labels on input plugs but not outputs (like a shader) or on output plugs but not inputs (like query nodes). It does make it a bit more verbose so happy to discuss if it would be better to settle on something simpler.

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.

[ericmehl]

Just noting I pushed an amendment that fixes the merge conflict, and makes a minor cosmetic change to a local variable name for consistency.