[🎨 Design] Add existing child to child widget future state

Goal

Allow users to add existing children to a child widget in order to speed up their workflow and reduce duplication.

Proposal

• Users can choose to add an available child from the Add dropdown #363452[child_options.png]
• Clicking to Add a child opens a title input in a focus state #363452[default_adding.png]
• As the user types and searches for the title of the child they want to add, they see existing children that match what they have inputted in a dropdown #363452[Searching_for_child.png]
• On click of an existing child within the search dropdown, the child is attached to the input and can be either cleared or added to the child widget #363452[Existing_child_attached.png]
• Assuming the child does not belong to another parent - The child will be added and a new "add" input opens in a focus state to speed up the user's workflow when adding items. #363452[existing_child_added.png]