Previous button on first step of the wizard in a dialog

I noticed that the wizard / dialog example would show the previous button on the first step of the wizard. Clicking it was producing an "Index out of Range" error.

Commit notes:
The _updateButtons function uses a :visible selector to determine which buttons should be available. However, when jWizard is applied to a dialog that hasn't been opened, none of the steps are visible yet. That case should be treated like the "first page" case.