Introduce global and scope/context-aware ClipboardHandler and SelectionHandler

Much like I introduced KeyMapper (change to KeyboardHandler? or KeydownHandler) Svedit should have just a single handler to mange selection changes or clipboard events (copy, cut, paste), this handler must be aware of the current context, so just like the KeyMapper track a scope_stack, so we have an app on top level and once we focus a Svedit instance it registers a new svedit-aware scope to take over the handling.

In other words I should get rid of these global event handlers in Svedit.svelte:

<svelte:document {onselectionchange} {oncut} {oncopy} {onpaste} />

Example scenario:

You have an app, that has some selection & copy&paste handling on app level, when no svedit instance is active. E.g. think of a side-panel that manages website design parameters, e.g. Fonts. And then when when you paste a font into that area, you should be able to hook into that paste handler, while any Svedit instance's handler should do nothing (as they are not in focus).

Start by reading Svedit.svelte and the existing KeyMapper implementation, especially how its scope_stack is managed. Replace the document-level selection and clipboard handlers with a global, scope-aware design; done means app-level handlers work when no Svedit instance is active and the focused Svedit scope takes over.