Move PaneContainerView::confirmClose to the model layer
Mock out document.title with Object.defineProperty


Previously the WorkspaceView::setTitle was mocked in specs to prevent
the title from changing. But I would like to move the title update logic
without breaking assertions, so now we can assert directly on
document.title.
Maintain document.title in the workspace model, not the view
Update document edited status in workspace model

This also fixes a previous oversight where the status wasn’t updated
when switching between pane items with different modified status.