Only tokenize in background if when a buffer is visible in an editor

This will prevent running a bunch of background tokenization when Atom is reloaded. We only perform tokenization to support content that is actually on screen.

src/app/editor.coffee

@@ -422,6 +422,7 @@ class Editor extends View
       @activeEditSession.off()
 
     @activeEditSession = @editSessions[index]
+    @activeEditSession.setVisible(true)
 
     @activeEditSession.on "buffer-contents-change-on-disk", =>
       @showBufferConflictAlert(@activeEditSession)