memory leak in Keyboard.js

[kalthoff]

In afterContentAdded() there are aspect.after() calls where the handles are never remov()ed. A few lines later other handles with on() are push()ed to grid._listeners grid._listeners.push(). What about push()ing the aspect.after handles there, too, to get them remove()d when the list is destroy()ed.

[dylans]

@kalthoff seems reasonable... do you have interest in creating a PR per the contributing guidelines?

[edhager]

Fix landed in commit 8af200e89fade841f3864df28a1b24499268270a.