Event annotations, API and chart UI for time series markup B)

[goodboy]

This has been a todo for quite some time and recently we've had a lot of
lurkerz and even some of our devs ask about it.

It's a feature that is like it sounds, a subsys for arbitrary annotation
of our charts from time-based events (normally stuff from the nooz).

there are plenty of existing charting platforms that have this for
reference:

• tinaview
• webull
• ib's tws

---

where to start in the code base:

• we already have a time-slicing api in:
  https://github.com/pikers/piker/blob/master/piker/data/_pathops.py#L294
  (which was implemented as part of getting multi-timeseries overlays
  working.
• we already have a chart annotation API as part of our order mode
  system:
  • order mode usage: https://github.com/pikers/piker/blob/master/piker/ui/order_mode.py#L537
  • actual graphics apis: ArrowEditor: https://github.com/pikers/piker/blob/master/piker/ui/_editors.py#L57

We mostly just need a public API for unifying these systems and
obviously some helper routines for parsing some input to its timestamp and content
and then displaying it with some set of options for graphical shapes to
draw.

Probably much more to come in this writeup B)