Improve extensibility of the DiagramWidget

[pbrown12303]

These changes improve the extensibility of the DiagramWidget. Changes include:

• The ability to add a background canvas object to a diagram
• The ability to add user-defined connection pads to diagram elements
• Addition of AnchoredText callbacks to facilitate coordinating changes to the anchored text

[coveralls]

Pull Request Test Coverage Report for Build 7492839566

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.9%) to 46.941%

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Totals
Change from base Build 7373224328:	-0.9%
Covered Lines:	1895
Relevant Lines:	4037

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💛 - Coveralls