The `ServerWidget` didn't validate the input of server_e, which allowed
the user to enter an invalid server connection string and exit the
dialog without knowing they entered an invalid string.
In the wizard this behavior is very misleading as the user explicitly
wanted to use a custom server, can click next after entering an invalid
server, and we will just silently fall back to automatic server selection.
This change will disable the "Next" button in the wizard if an invalid
address has been entered and show a red background in the server_e while
the input is not valid, so the user clearly sees that this input is not
going to be used.
f321x
c3d1b2046a