Implement a way for components to have placeholders

[mkhatib]

This can basically be used in-place of the text placeholders for Paragraph but also allow non-text components to have placeholders.

For example, A Figure component can have a default image as a placeholder that allows users to click on it to upload the Figure source and replace that placeholder with something else.

This would also open the door for users to create a template for an article and allow users to "fill" it.