API: contextMenu

[susannemoog]

We need an API endpoint to get the ContextMenu for an item (see also pagetree implementation of context menus) with according access/permission checks and based on the current context.

The single items should have callbacks attached to trigger according actions.

[lolli42]

three entry points done as shown in screens on #33, call back actions are prepared and should be filled if js side knows what should be put in there. some items may need additional data atributes, this can be added by js implementors if needed.