added (get)lastVisibleRow to editor.dart and virtual_renderer.dart

@rmsmith - We needed to add (get)lastVisibleRow.

I began to implement tests for this, but it looks like a mock virtual renderer would have to be added to the tests (`getLastVisibleRow` currently always reports 0 with the default renderer).  What do you suggest?  Have you attempted using the mock renderer from the ace.js tests?