Introduce slot concept to control visibility and positioning of visual extensions

[nikku]

Context

With many extensions added to a diagram the positioning of elements gets messy:

As we are an open framework and agnostic of extension use-cases we cannot not impose concrete positioning ("fix the mess"), however we can provide the necessary structures to display extensions on the canvas and/or particular elements in an orderly manner.

Describe the solution you'd like

• Identify places on the diagram that extension authors can render into in a controlled manner ("slots").
• Provide an API to render into these slots
• Allow slots to be tagged so that they can be toggled on and of, showing or hiding rendered contents.

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Potential slots:

• Canvas: top-left (palette), top, top-right, bottom-*
• Per element: top-left, bottom-right
• ...