Add "Trim Whitespace" to the sandbox.

Ok, so I had a few goals here:

1. Put `Trim Trailing Space` on steroids so that it also trimmed extra newlines at the end of files.

2. Preserve existing features like "respect the preference for trimming empty lines" and "preserve spaces on the line where the cursor is"

3. Do it in the sandbox.

4. Rip `Trim Trailing Space` out of the core.

So I did 1 through 3, but it wasn't clear to me how to do 4. Also I want to preserve the preference for `Toggle Trim Trailing Space On Save` to apply to the new `Trim Whitespace` feature instead, but I wasn't sure how.

Any advice here?