fix(core/group): show context-menu based on dropdown

[danielleroux]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build (yarn build) was run locally and any changes were pushed
• Unit tests (yarn test) were run locally and passed
• Visual Regression Tests (yarn visual-regression) were run locally and passed
• Linting (npm lint) was run locally and passed

Pull request type

Please check the type of change your PR introduces:

• Bug fix
• Feature
• Refactoring (no functional changes, no API changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

GitHub Issue Number: #152

What is the new behavior?

• Use dropdown in slot to display context-menu

Does this introduce a breaking change?

• Yes
• No

[nuke-ellington]

Can we improve the dropdown placement in relation to the button/header?
I cannot find a refernce in the styleguide but it looks a little off to me :)

[danielleroux]

@nuke-ellington
It is off because of the popperjs implementation. Its fixed in #137 (#153 )