Remove Render() from ListContextTrait.HandleRender

self.c.Render() at the end of HandleRender was there to schedule a gocui Update
tick so the view content modified above would actually get drawn. For UI-thread
callers (the great majority -- keybinding handlers, the layout function itself,
popup resize, etc.) this was unnecessary work, since gocui already runs a
layout/redraw cycle after every event. SimpleContext.HandleRender doesn't call
Render() either, so this aligns the two implementations.

The few callers that drove HandleRender from a worker goroutine and relied on
Render() for the flush were wrapped in OnUIThread in the preceding commits, so
the implicit Render is no longer needed.

Also, Render() being called *before* setFooter() looks like it might have been a
theoretical race; this is no longer an issue now.

pkg/gui/context/list_context_trait.go

@@ -124,7 +124,6 @@ func (self *ListContextTrait) HandleRender() {
 		content := self.renderLines(-1, -1)
 		self.GetViewTrait().SetContent(content)
 	}
-	self.c.Render()
 	self.setFooter()
 }