message: cmake: fix plugin getter name
  This changes autogenerate_plugin_container.cmake script to generate the
  plugin getter method to be named with an underscore as you would expect.
  For instance for the plugin FollowMe this would yield `follow_me`
  instead of `followme`.
diff: @@ -119,7 +119,13 @@ set(PLUGIN_LIST_APPEND_STRING "")
  # Go through all the plugins and generate the strings.
  foreach(class_name ${plugin_class_names})
  - string(TOLOWER ${class_name} class_name_lowercase)
  + # We want to go from CamelCase to snake_case.
  + # CamelCase -> Camel_Case
  + string(REGEX REPLACE "(.)([A-Z][a-z]+)" "\\1_\\2" class_name_with_underscores "${class_name}")
  + # Camel0Case -> Camel0_Case
  + string(REGEX REPLACE "([a-z0-9])([A-Z])" "\\1_\\2" class_name_with_underscores "${class_name_with_underscores}")
  + # Camel_Case to snake_case
  + string(TOLOWER ${class_name_with_underscores} class_name_lowercase)
  set(get_name ${class_name_lowercase})
  set(member_name "_${class_name_lowercase}")
  