Cleanup: Move status bar code out of EditorManager into a new module

Keeps EditorManager tightly focused on maintaining the set of Editors and gives the status bar code room to grow for future UI additions.

This removes EditorManager._init() and the manual calls to it from unit tests, which seemed unnecessary. EditorStatusBar._init() is still called on htmlReady -- just not manually called by any unit tests.

Also changes the status bar code to use active editor more consistently (used to use focused editor sometimes).  That should be the only diff in the big chunks moved out of EditorManager.