Add 'Remember' CheckBox to the LoginDialog

[manekovskiy]

This adds a "Remember ..." checkbox to the LoginDialog. Reference feature #2305.

To keep backward compatibility the Checkbox is Collapsed by default.
The default text for Checkbox equals to "Remember".

[michaelmairegger]

I would change the name of this property to a more boolean-sounding name. I would suggest something like IsRememberRequested, RequestRemember. This is because the developer might be confused because it might understand that MahApps does remember the state of the dialog.
Another possibility would be to add a documentation tag to the property.

[thoemmi]

I think ShouldRemember is better since it's a request to the caller to persist the data.

Additionally I'd suggest to make all setters of LoginDialogData internal. There's no need to set these properties outside of MahApps.Metro.

[manekovskiy]

Yes, I was also thinking about ShouldRemember as alternative to Remember. It is more explicit.
Totally agree on suggested internal improvement.