Annotation Improvements

[ericmehl]

This adds a few enhancements to node annotations :

• Copy annotation from a new right click context menu
• Paste annotation onto the selected nodes using Control + V
• Double click annotation to pop up the annotation editor for it
• Intrepid users can customize the right click menu by connecting to AnnotationsUI.contextMenuSignal() or get individual annotations from the graph editor-wide annotations gadget using AnnotationsGadget::annotationAt().

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]

Thanks Eric. I like the overall approach with annotationAt() doing the heavy lifting in C++ and then everything else being done on top in Python. And it feels great to be interacting more directly with the annotations in the UI now. Specific comments inline as usual...

[ericmehl]

I have the comments addressed and noted above. There's a couple of additional commits as well :

• bfb53e0 : Inherits the color from a templated annotation for later pasting.
• 1d8a888 : Fixes an issue where double clicking / right clicking on the bookmark annotation may allow you to copy a text annotation in error. The diff on Github is weird, all that's actually happening is putting the bookmark and numbered bookmark drawing inside a conditional that checks to see if selectionIds is given (and not drawing if they are).

Ready for a new look!

[johnhaddon]

Thanks Eric, LGTM! Could you squash down ready for merging please?

[ericmehl]

Squashed and merging!