Updater (Swing) Sites Dialog: fix editing fields

This is a proper fix for the issue that 18c42e5c(Updater (Swing) Manage
Update Sites: only update String values after editing) tried to address:
after hitting the Enter key in fields, under certain circumstances, the
setValueAt() method was called twice: once while still editing and once
after editing.

However, it seems that the setValueAt() method is no longer called twice
always, so we have to be cleverer now and perform the desired actions only
when setValueAt() wants to actually change the value.

Noticed by Ignacio Arganda-Carreras.

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