UI Framework: WizardApplication - More flexible buttons

[Moises Henriquez]

When using a WizardApplication object, at the very last step the buttons at the bottom of the UI are hardcoded and labeled "Quit", and "Reboot" which may not always be what a developer needs.

https://bitbucket.org/VLCore/vinstall/src/282c4fc2306cfae0983743fc8113368c39f6f2d1/vinstall/core/wizard.py?at=master&fileviewer=file-view-default#wizard.py-147

There should be a way to change these from the Controller side. Maybe the default should even be one single button labeled 'Done' or 'Quit', and offer the ability to change the label there or even add another button.