Add virutal QWizard:: and QWizardPage::previousId() functions to support dynamic page ordering whe going backwards

QWizard and QWizardPage has virtual nextId() functions to support dynamic page ordering when clicking to go to the next page in the wizard. However, similar functions does not exists for when going back a page in the wizard.

It would be nice if QWizard and QWizardPage had virtual previousId() functions to support dynamic page ordering when going back a page as well.

Sample use case:

Consider the following three pages in a wizard:

1. Enter personal information (name, address etc.).

2. Confirm information. Show entered information and ask for confirmation. Click next to
confirm and back to change the previously entered information.

3. Some other page.

Clicking back on page 2 should take the user to page 1 (which currently happens).

Clicking back on page 3 should not take the user back to page 2, but to page 1 (which currently is not supported).