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
events.

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
  goroutine
- 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
  callbacks

The exact goroutine requirements for Drawable have been documented.

2 files changed, 7 insertions, 4 deletions

diff --git a/lib/ui/interfaces.go b/lib/ui/interfaces.go
index d27afe2..0cdffc1 100644
--- a/lib/ui/interfaces.go
+++ b/lib/ui/interfaces.go
@@ -4,12 +4,15 @@ import (
 	"github.com/gdamore/tcell"
 )
 
+// Drawable is a UI component that can draw. Unless specified, all methods must
+// only be called from a single goroutine, the UI goroutine.
 type Drawable interface {
-	// Called when this renderable should draw itself
+	// Called when this renderable should draw itself.
 	Draw(ctx *Context)
-	// Specifies a function to call when this cell needs to be redrawn
+	// Specifies a function to call when this cell needs to be redrawn. The
+	// callback may be called in any goroutine.
 	OnInvalidate(callback func(d Drawable))
-	// Invalidates the drawable
+	// Invalidates the drawable. This can be called from any goroutine.
 	Invalidate()
 }
 
diff --git a/lib/ui/ui.go b/lib/ui/ui.go
index 9d9a5da..91a26da 100644
--- a/lib/ui/ui.go
+++ b/lib/ui/ui.go
@@ -10,7 +10,7 @@ import (
 
 type UI struct {
 	Content DrawableInteractive
-	exit    atomic.Value
+	exit    atomic.Value // bool
 	ctx     *Context
 	screen  tcell.Screen