Create plugins developer guide (#66)

* Add Type Modules Plugin info in developer guides

* Add Libpeas Plugin info in developer guides

source/developer-guides/plugins.rst

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+Plugins
+=======
+
+.. toctree::
+   :glob:
+   :maxdepth: 1
+
+   plugins/*

source/developer-guides/plugins/01-type-modules.rst

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+Type Modules
+============
+
+This example shows you how to implement a GLib.TypeModule based plugin in Vala. It also shows the usage of static construct/destruct block.
+
+.. code-block:: vala
+
+   // plugin.vala
+   public class MyClass : Object
+   {
+           static construct
+           {
+                   message("MyClass init");
+           }
+
+           static ~MyClass()
+           {
+                   message("MyClass deinit");
+           }
+   }
+
+   [ModuleInit]
+   Type plugin_init(GLib.TypeModule type_modul)
+   {
+           return typeof(MyClass);
+   }
+
+
+.. code-block:: vala
+
+   // loader.vala
+   class MyModule : TypeModule
+   {
+         [CCode (has_target = false)]
+         private delegate Type PluginInitFunc(TypeModule module);
+
+         private GLib.Module module = null;
+
+         private string name = null;
+
+         public MyModule(string name)
+         {
+                  this.name = name;
+         }
+
+         public override bool load()
+         {
+                  string path = Module.build_path(null, name);
+                  module = Module.open(path, GLib.ModuleFlags.BIND_LAZY);
+                  if(null == module) {
+                           error("Module not found");
+                  }
+
+                  void * plugin_init = null;
+                  if(! module.symbol("plugin_init", out plugin_init)) {
+                           error("No such symbol");
+                  }
+
+                  ((PluginInitFunc) plugin_init)(this);
+
+                  return true;
+         }
+
+         public override void unload()
+         {
+                  module = null;
+
+                  message("Library unloaded");
+         }
+   }
+
+   // Never unref instance of GTypeModule
+   // http://www.lanedo.com/~mitch/module-system-talk-guadec-2006/Module-System-Talk-Guadec-2006.pdf
+   static TypeModule module = null;
+
+   int main()
+   {
+         module = new MyModule("plugin");
+         module.load();
+
+         var o = GLib.Object.new(Type.from_name("MyClass"));
+
+         // free last instance, plugin unload
+         o = null;
+
+         return 0;
+   }
+
+**Build**:
+
+.. code-block:: console
+
+   $ valac -o loader loader.vala --pkg=gmodule-2.0
+   $ valac --ccode plugin.vala
+   $ gcc -fPIC -shared -o libplugin.so plugin.c $(pkg-config --libs --cflags gobject-2.0 gmodule-2.0)
+
+**Run**:
+
+.. code-block:: console
+
+   $ LD_LIBRARY_PATH=$PWD ./loader
+
+Which should output something like this:
+
+.. code-block:: output
+
+   ** Message: plugin.vala:5: MyClass init
+   ** Message: plugin.vala:10: MyClass deinit
+   ** Message: loader.vala:37: Library unloaded

source/developer-guides/plugins/02-libpeas.rst

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+Libpeas
+=======
+
+In advanced scenarios, you may need to create a plugin engine to manage plugins in your application.
+
+You can save time by using Libpeas, a library that handles that logic of making your application extensible with plugins.
+
+**Links**:
+
+* `Project Repository <https://gitlab.gnome.org/GNOME/libpeas>`_
+* `libpeas-2 - Valadoc Package page <https://valadoc.org/libpeas-2/index.htm>`_
+* `libpeas-2 - C Documentation <https://gnome.pages.gitlab.gnome.org/libpeas/libpeas-2/>`_