Any row additional/removal marks the tracked row data as dirty. This
catches `someprop: some_mode[42];` being marked as dirty when a row
before 42 is inserted/removed (which makes sense). It'll also mark it as
dirty when a row after that is added/removed, but that keeps the code
simple for now.
This is done by exposing the ModelNotify to the caller via the new
ModelTracker trait, which has a function that allows "hooking" into the
dirty tracking of the size.
By extension, this also works in JavaScript.
cc #98
* Change SharedVector::as_mut_slice() to SharedVector::make_mut_slice()
* SharedPixelBuffer::as_mut_slice and SharedPixelBuffer::make_mut_slice()
* SharedPixelBuffer::as_bytes_mut() to SharedPixelBuffer::make_mut_bytes()
This is consistent with Rc::make_mut() and is supposed to emphasize
that, unlike the usual as_* operations, it is not a free operation but
may involve work (detaching the data by cloning it).
Amends commit 7384ebdc32
