[editing] revise lsp editing implementation

[JanKoehnlein]

The current editing implementation based on LSP puts to much LSP magic on the client side. This leaks some abstractions, e.g. how to interpret traces. Into the bargain, it requires to relay language server requests between webview and extension, which was not necessary in Theia where there was no separate webview app.

• Move as much as possible of the editing logic to the language server.
• Use the new inline editing functionality of sprotty to edit rename labels
• Maybe use new popup UI to edit cross-reference labels
• Implement DeleteAction