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
log refusal to serve hidden files dirs
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
fix test suite

is_hidden expects the file to actually exist, so I've rearranged the
logic such that is_hidden is called only after it's clear that the file
exists.