In the SE toolbar buttons: passsed the click event to the handlers

What does this MR do and why?

Extends the Source Editor buttons with the ability to pass on the original click event to the event handlers. When working with the SE toolbar buttons it might be necessary to access the button itself to get some additional information about the clicked button (when generalizing the click handlers), like data-* attributes. Hence the change.

• This is the first preparation step for #352662 (closed).
• The next will be to extend the button component to accept the data-* attributes.
• The last step in implementing Migrate actions from Markdown toolbar to Source... (#352662 - closed) will be the migration of the Markdown icons with the use of both of these changes.

Screenshots or screen recordings

No visual change

How to set up and validate locally

The whole toolbar is behind the :source_editor_toolbar feature flag. This particular MR doesn't contain the use implementation yet and serves as the preparation step for the actual implementation.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #352662 (closed)