Do not prompt username/password if already provided

[jornane]

If a username/password combination is already provided in the ClientSideCredential, do not prompt for a username/password.

<ClientSideCredential>
	<UserName>user@example.com</UserName>
	<Password>Tr0b4d0r</Password>
</ClientSideCredential>

[jornane]

This fixes the most pressing problems in #167.

However, it does not handle the case where a UserName was provided but not a Password.

[SanderVeer]

This fixes the most pressing problems in #167.

However, it does not handle the case where a UserName was provided but not a Password.

Thanks for the head start on this issue Jørn! I made a slight tweak to your code but it should work as intended now.

The issue regarding not pre-filling the username if one is provided, but no password, still stands though. This wasn't part of the original implementation of the app but I'll definitely add it to the list of potential areas of improvement.