[2.0.0-beta3] Clean src/ folder ?

Hi,

As I am not following closely UI Patterns 2.x, I don't know what SchemaGuesserInterface.php and WithJsonSchemaInterface.php are but by their names if I may propose the following organisation:

• Annotation/
• Element/
• Form/
• Plugin/
• PropTyping/
  • PropTypeAdapterInterface.php
  • PropTypeAdapterPluginBase.php
  • PropTypeAdapterPluginManager.php
  • PropTypeInterface.php
  • PropTypePluginBase.php
  • PropTypePluginManager.php
• SchemaManager/
  • SchemaGuesserInterface.php
  • WithJsonSchemaInterface.php
• Source/
  • SourceInterface.php
  • SourcePluginBase.php
  • SourcePluginManager.php
• Template/
• ComponentPluginManager.php
• PluginSettingsInterface.php

And also if you want split PropTyping into a PropTypeAdapter and a PropType folders.