Looking for sample plugin code to help create custom UI builder

Would like to build a GUI designer/builder for custom file type.  A user would draw their components and the plugin would build the associated declarative language, which would then eventually feed into a build step.  I'm new to plugin development.  Looking to see if either the IntelliJ GUI builder code is open source or if anyone can point to examples or documentation to:

Create the skeleton or hooks to a graphic file editor as opposed to a text editor and associate it with a file extension, such  '.gtemplate'.

Add toolbar icons/actions that only appear when new graphic editor plugin is in use.

Add support to create new graphic template file in standard Java project.

Thanks.