DYN-6442 Update Dropdown node population method

[QilongTang]

Purpose

This PR fixes two bugs

1. Once graph with the custom selection node is opened, the DYN graph is marked as dirty -> has unsaved changes
2. Once items are populated from the custom selection node, the DYN graph is marked as dirty - > has unsaved changes.

I was trying to write a unit test for this change but I found the behavior for PopulateItems() is different when calling from user interaction v.s. Dynamo view level unit test which would affect selectedIndex change. So this change will need to rely on the test complete test created by @avidit

Declarations

Check these if you believe they are true

• The codebase is in a better state after this PR
• Is documented according to the standards
• The level of testing this PR includes is appropriate
• User facing strings, if any, are extracted into *.resx files
• All tests pass using the self-service CI.
• Snapshot of UI changes, if any.
• Changes to the API follow Semantic Versioning and are documented in the API Changes document.
• This PR modifies some build requirements and the readme is updated
• This PR contains no files larger than 50 MB

Release Notes

fixes two bugs

1. Once graph with the custom selection node is opened, the DYN graph is marked as dirty -> has unsaved changes
2. Once items are populated from the custom selection node, the DYN graph is marked as dirty - > has unsaved changes.

Reviewers

FYIs

@DynamoDS/dynamo