Line folds are managed using metadata of AssDialogue elements, and
saved in the project properties. They only affect the appearance of the
subtitle grid, and have no impact on other line operations.
Use explicitly defaulted destructors for base cases when possible,
eliminate aggregate initialization constructors where possible, and push
some more stuff to NSDMIs.
ptr_vector hasn't been updated for C++11, so despite being specifically
designed to store pointers to objects it's less safe and not really any
easier to use than a regular vector of unique_ptrs
The grid needs to be able to map AssDialogue * to a row number, and just
storing it in the AssDialogue is nearly strictly better than a std::map
mapping events to rows. Probably will be of use elsewhere as well.
arch1t3cht