Autoscroll viewport to keep selected sexp visible

[mkremins]

When the selection moves from one sexp to another, Flense should check to see if the newly selected sexp is visible in the current viewport. If it is not, the newly selected sexp should be automatically scrolled into view.

[mkremins]

The new token-stream rendering model (see #15) has introduced a regression here. This functionality will need to be reimplemented.

[mkremins]

Consider using Element.scrollIntoView for this.