Do dynamic input validation in the startup dialogs

[jrick]

We can display a red border (or do even fancier things) around various text boxes where we know the input is already invalid, instead of opening a error message box when trying to continue to the next part of the app. For example, the network address for the dcrd RPC instance and seeds when recovering or confirming a new wallet backup can be validated in real time.

When any of the input is predetermined to be invalid, the button to continue through the dialog should be disabled.