Implement JS container to store notebook-wide style and variables

[disberd]

The various extra widgets from PlutoExtras often come bundled with a significant amount of CSS.

This is probably not a big issue in usability but it becomes tricky when one wants to try modifications of CSS for a specific widget type in the developer console, as the same CSS is applied multiple times (one for each instance of the object hardcoding the CSS).

It would be cleaner to have some functionality in PlutoDevMacros to eventually add/modify style sheets for specific groups of objects (e.g. classes) that does duplicate style sheets.

The idea is to create a small JS container within the HTML body and always put stuff there, being careful on replacing stuff instead of duplicating it so that a single style is present

[disberd]

Closing this as we are not going to implement the js functionality in this package