create a common base class for ASSString and ASSLineTextSection
support metatable inheritance in the class factory and add a meta even registration function to for classes; add metatable event for "less than" to classes based on ASSNumber
add support for undoing the previous line commit
add method for checking whether a given line is animated (changes during its duration) or not
add whole-line bounding box support to ASSLineContents:getMetrics(), provided by ASSInspector
fix typo