Remove methods taking `&mut Workspace` from `Pane` (#2540)

This pull request simplifies the `Pane` struct by replacing methods like
`Pane::add_item` that would previously take a `&mut Workspace` with
methods that take a `&mut self`. When access to the workspace is needed,
we now either emit an event from the `Pane` or directly move the method
to the `Workspace` struct.