LuaAssFile relied on the lifetime guarantees of intrusive lists (i.e.
you can pretty much do whatever the fuck you want and it'll work out),
but AssInfo objects are no longer owned by an intrusive list. Do awful
things to lazily copy the header section when needed to provide similar
lifetime guarantees.

It has zero benefit and forces an extra copy.

Eliminates a bajillion dynamic casts.

Apparently flushing immediately after setting the clipboard sometimes
doesn't work.

They're only used by one thing (the drag visual tool), so calculating
them is pointless overhead most of the time, and it simplifies the
visual tool code much less than it complicates the grid code.

Store the data in vectors rather than AssFiles since even an intrusive
linked list has comically high memory overhead. Cuts memory usage of a
full undo stack with 15k lines by 65 MB for 32-bit and 130 MB for
64-bit. Also roughly halves how long it takes to copy the file for the
undo stack, and makes undo/redo a bit faster.

Make them significantly smaller to cover less of the video and fiddle
with the appearance to hopefully make them still visible enough.

Passing a codepage makes libass convert to UTF-8 even if it's already
UTF-8.

Despite being completely unmentioned in the documentation this appears
to be required to show up in the default programs UI on Windows 8.1.

Closes #1699.

These weren't merged from 2.1.9 with the rest of the file association
code, which results in the installer trying to reference nonexistent
resources and things breaking.

Updates #1699.