Use ACTION_* constants instead of string literals

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Esthetic changes

  * _onPageReady should not expect event argument
  * signal handler have the first argument named emitter by convention
  * update description of ratingOptions' fields
  * Add doc comment Base/custom actions

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Fix reference error ACTION_ALERT

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>