Start of new callback system
Add mock to requirements for testing
Add docstrings for callbacks
Cross link signatures docstrings
Add tests for callback infrastructure
Deprecate some hooks in favour of callbacks
Use callbacks system for autoreload
Document new callbacks system
Rename callbacks -> events (mostly), fire -> trigger
Add note about experimental API
Rename pre/post_execute_explicit events to pre/post_run_cell
Rename shell_initialised for orthographically challenged English speakers
Clarity fixes pointed out by @ellisonbg
Update docstrings to refer to ip.events
Use silly American spelling for initialised
callbacks -> events in docs
Use new events API for inline backend flush_figures
Mention silent execution in docstrings
Update comment
Fix wheel dependency specification