notification: add gui for sendmail notification endpoints

notification: add gui for sendmail notification endpoints

This commit adds a new panel 'NotificationConfigView' that is supposed
to be embedded in the datacenter configuration side-bar.
This new view lists all notification endpoints, allowing to
add/modify/delete/test them.

Furthermore, this commits adds the dialog for adding/modifying
sendmail endpoints. The dialog is 'plugin-in' based, meaning that it
consists of a base window (EndpointEditBase) and a panel that holds
the actual fields for the endpoint type configuration. This will show
be beneficial once the GUI for other endpoint types is added.

Signed-off-by: Lukas Wagner <l.wagner@proxmox.com>
Reviewed-by: Dominik Csapak <d.csapak@proxmox.com>