Simplify EPlugin loading at startup.

- Require all EPlugin and EPluginHook subtypes be registered before
  loading plugins.  This drastically simplifies the EPlugin/EPluginHook
  negotiation.

- Turn most EPluginHook subtypes into GTypeModules and register their
  types from an e_module_load() function (does not include shell hooks).

- Convert EPluginLib and the Mono and Python bindings to GTypeModules
  and register their types from an e_module_load() function, and kill
  EPluginTypeHook.

modules/plugin-python/example/org-gnome-hello-python-ui.xml

@@ -0,0 +1,16 @@
+<Root>
+  <commands>
+    <cmd name="HelloPy" _label="Hello Python"
+      _tip="Python Plugin Loader tests"
+      />
+  </commands>
+
+  <menu>
+    <placeholder name="MessagePlaceholder">	  
+      <submenu name="Message">
+	  <separator f="" name="sep"/>
+	  <menuitem name="HelloPy" verb=""/>
+      </submenu>
+   </placeholder>
+  </menu>
+</Root>