Ability for plugins to add custom CSS/JS

Currently, the only way to customize the is by themes (colors).

Adding an option to inject custom CSS and/or JS code from a plugin into the main window would allow to customize the UI freely, to make it more personalized and easier to use.

A couple of examples include: changing sizes of controls, enabling deleting headers/params by double click (instead of the dropdown menu) or additional actions for various UI elements.