Remove Image -> Isometric menu

In the future  I may add this feature back as a plugin of some sort, but
right now it would require an unfortunate amount of of work to make it
layers-friendly (and given its niche appeal, I'd rather spend that time
elsewhere).
Get Image -> Crop to Selection working with layers

I've been dreading this, but it ended up being a great chance to revisit
some old code, written before PD switched to its current "always
premultiply alpha" approach.  Because of that, I was able to not only
get Crop to Selection working with layers, but also to greatly improve
the performance of cropping in general, as well as retrieving a chunk of
the image as processed by a selection (yay!).

Note that cropping an image to a selection may cause some layers to
become fully transparent, if they lie off completely off the selection.
So it's not a bug if layers suddenly don't have thumbnails after the
operation.