Reorganize Seravo Plugin folder structure

[Moppa5]

Description As a part of the cleanup and Up-To-Date project one point was about Seravo Plugin folder structure. We should investigate and reorganize a proper and cleaner folder structure for Seravo Plugin. There are no clear plans yet for this as it's depending mostly on how the new implementation & design will be after the cleanup.

However, we can discuss and document here about the changes.

[JoosuaKoskinen]

@Moppa5 and I decided the folder structure for postboxes to be the following:

/lib
   |
   |__ /postbox
   |          |__ postbox.php
   |          |__ toolpage.php
   |          |__ handler.php
   |          |__ /component
   |          |            |__ component.php
   |          |            |__ templates.php
   |          |__ /templates
   |                       |__ auto_command.php ....
   |
   |__ /ajax
   |       |_____ handler.php
   |       |_____ response.php
           |_____ /templates
                           |__ auto_command.php ....