Is your feature request related to a problem?

Make all elements stylable with CSS.

Describe the solution you'd like

This is a placeholder to collect all PR's that deal with making the styling capabilities of Gaphor (a lot) better.

The property editor can contain a section where the style node structure is shown.

For a Class that could look something like:

  class
   ├╴stereotypes
   ├╴name
   ├╴from
   ├╴compartment
   │  ├╴attribute
   │  ╰╴attribute
   ╰╴compartment
      ╰╴operation

Describe alternatives you've considered

There's no alternative. CSS is the way.

Additional context

Ideally we get rid of all hard-coded ("inline") styles.