Squashed commit of the following:

commit 48eb4d8839
Author: Syrone Wong <wong.syrone@gmail.com>
Date:   Sun Sep 18 15:46:09 2016 +0800

    Drop useless interface impl

    Signed-off-by: Syrone Wong <wong.syrone@gmail.com>

commit c0a4f3421a
Author: Logan Zhou <xorcererzc@gmail.com>
Date:   Sun Sep 18 00:58:02 2016 +0800

    Revert "Updated to meet the hint from Wongsyrone."

    This reverts commit 84c8053577.

commit 84c8053577
Author: Logan Zhou <xorcererzc@gmail.com>
Date:   Sun Sep 18 00:52:21 2016 +0800

    Updated to meet the hint from Wongsyrone.

commit 333ccb6d1f
Author: Logan Zhou <xorcererzc@gmail.com>
Date:   Sun Sep 18 00:34:52 2016 +0800

    Added duplicate button to config form.

Signed-off-by: Syrone Wong <wong.syrone@gmail.com>

User wants to use that server after selected it in the ConfigForm. If user deletes
the server in use, we can't find its index, then the previous logic will revert to
index 0, which is not expected behaviour.

Since we handled 'ServersListBox.SelectedIndex' in various event handlers, we can
trust it all the time.

Signed-off-by: Syrone Wong <wong.syrone@gmail.com>

- add event handler for KeyDown
- drop unused code, we don't support
  table and rc4 now

Signed-off-by: Syrone Wong <wong.syrone@gmail.com>

Signed-off-by: Syrone Wong <wong.syrone@gmail.com>

fix ArgumentOutOfRangeException

Sometimes the users may hit enter key by mistake, and the form will
close without saving entries.

2 fixes, 1 feature, 1 code beautify

Change the first letter of function to upper case letter to fit C# code
style

Now can use esc key to cancel, and don't change focus after change
server from list

Add a lock on config form to avoid accidental changes