Start work on plugin framework for editor

src/calibre/customize/__init__.py

@@ -741,3 +741,8 @@ class ViewerPlugin(Plugin):  # {{{
         pass
 
 # }}}
+
+class EditBookToolPlugin(Plugin):  # {{{
+    pass
+# }}}
+

src/calibre/customize/ui.py

@@ -8,7 +8,7 @@ from calibre.customize import (CatalogPlugin, FileTypePlugin, PluginNotFound,
                               MetadataReaderPlugin, MetadataWriterPlugin,
                               InterfaceActionBase as InterfaceAction,
                               PreferencesPlugin, platform, InvalidPlugin,
-                              StoreBase as Store, ViewerPlugin)
+                              StoreBase as Store, ViewerPlugin, EditBookToolPlugin)
 from calibre.customize.conversion import InputFormatPlugin, OutputFormatPlugin
 from calibre.customize.zipplugin import loader
 from calibre.customize.profiles import InputProfile, OutputProfile
@@ -527,6 +527,13 @@ def all_viewer_plugins():
             yield plugin
 # }}}
 
+# Editor plugins {{{
+def all_edit_book_tool_plugins():
+    for plugin in _initialized_plugins:
+        if isinstance(plugin, EditBookToolPlugin):
+            yield plugin
+# }}}
+
 # Initialize plugins {{{
 
 _initialized_plugins = []