Add optional warning message when clicking on back/continue to wizard

[Seeloewen]

For some pages, it would be great to have a warning when going back or continuing, because the user might lose progress made or something similar. This warning will probably be a yes/no messagebox.

Most likely implementation in class WizardPage:

bool hasBackWarning;
bool hasContinueWarning;
string backWarning;
string continueWarning;