Update Sites: do not handle key events from just-closed dialog

We should give the user a chance to provide a password before we try
to log into the Wiki. But when the user hit "Enter" to dismiss the
message that a Wiki account was registered and needs to be activated,
the "key released" event happens already after the message dialog is
dismissed, at least on MacOSX. To avoid misinterpreting this event as
if it originated from the password field, let's handle the "key
pressed" event instead.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Update Sites: give the user a chance to log in

When an existing Wiki account is used to add a personal update site, we
should let the user log in... So let's enable the password field and
provide a helpful message in that case.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>