ref: 0.1.1 aerc/lib d---------
Add Tabs history

Fixes #77: When closing a tab, bring you back to the one you last had focused
ensureScroll on text input frames
Implement scrolling in text input
Show account wizard if no accounts configured
New account wizard, part one
Flesh out multipart switcher
lib/ui/ui: use atomic instead of channel

This makes it so an atomic `invalid` value is used instead of an unbuffered
channel. When many invalidations kick in, a lot of values were sent to the

(Since OnInvalidate's callback can be run in any goroutine, we need to be
careful about races here.)
Update internal state and draw from the same goroutine

This commit introduces a new Aerc.Tick function that should be called to
refresh the internal state. This in turn makes each AccountView process worker

The UI goroutine repeatedly refreshes the internal state before drawing a new
frame. The reason for this is that many worker messages may need to be
processed for a single frame, and drawing the UI is far slower than refreshing
the internal state. This has been confirmed in my testing (calling Aerc.Tick
only once per frame results in a slower display).

Many synchronization code has been removed. We can now write widgets without
having to care so much about races. The remaining sync users are:

- widgets/spinner: the spinner value is updated from inside an internal
- lib/ui/invalidatable: Invalidate may be called from any goroutine
- lib/ui/grid: same
- lib/ui/ui: an internal goroutine needs read access to UI.exit
- worker/types/worker: Worker.callbacks is used for both worker and UI

The exact goroutine requirements for Drawable have been documented.
Fix issues with OOB uids
Implement (basic form) of :reply
Let caller pass in custom headers to compose
Implement move, mv commands
Implement :copy (aka :cp)
Update tab name as subject changes

Also moves truncation to the tab widget
Remove leftover debug logging
Handle incoming emails gracefully
Spec out review message screen
Implement :{next,prev}-field in compose view
Add initial compose widget