Added a document about how we handle plugins and dynamic loading.

Original commit message from CVS:
Added a document about how we handle plugins and dynamic loading.

docs/random/plugins

@@ -0,0 +1,89 @@
+We describe how the plugin system works.
+
+Plugins
+-------
+
+plugins are basically shared libraries with a plugin_init
+function.
+
+they provide the GStreamer core library with the folowing
+information:
+
+  - elementfactories
+  - typefactories
+  - metadatafactories?
+
+ElementFactory
+--------------
+
+elementfactories provide the core library with elements (duh)
+
+an elementfactory has the following information:
+
+  - a unique name for the element
+  - strings describing the element (name, desciption, copyright,...)
+  - a description of the media types it accepts (as capabilities) 
+  - a description of the media types it outputs (as capabilities)
+
+
+TypeFactory
+-----------
+
+has the following properties:
+
+ - a mime type
+ - file extensions that may provide a hint for this type
+ - a function to detect this type
+ - a string to detect this type (file(1) like)
+
+
+XML registry
+------------
+
+The complete plugin tree will be exported into an XML desciption so
+that the definitions of the factories can be obtained without having
+to load and plugin_init the plugins.
+
+Loading of plugins
+------------------
+
+at some point, the plugin will need to be read into memory before
+any elements it provides can be used. the XML description of the
+loaded plugin will need to be updated.
+
+We will have the following methods for (implicitly) loading plugins:
+
+ gst_plugin_load_all()
+
+  remove the complete XML tree and read all the plugins in the
+  paths. The plugin_init method will be called and the XML tree
+  will be rebuild in memory.
+
+ gst_plugin_load (name)
+
+  The plugin will be located in the tree and if it allready loaded, the
+  function returns. If it is not loaded, the XML entry is removed and
+  the plugin is loaded. The plugin_init is called so that the XML tree
+  is reconstructed.
+
+ gst_elementfactory_create (factory, name);
+
+  The plugin providing the element will be located, if the plugin is
+  allready loaded, an element with the given name is created.
+  if the plugin is not loaded, gst_plugin_load is called.
+  the loaded factory is located again and the element is created.
+
+ The typefind function is called
+
+  When the plugin description  is read from the XML file, the typefind 
+  function is hooked up to a dummy typefind function. The dummy typefind 
+  function will locate the plugin it belongs to and call gst_plugin_load.
+  after the loading of the plugin, the real typefind function is called.
+
+
+
+ 
+
+  
+
+