Previously, whenever the user selects some text in MacVim, and then
bring up the Services menu (either by right-clicking on the selection,
or go to MacVim -> Services), MacVim will copy the selected text to the
system clipboard, which is quite an unexpected behavior. Fix that here.

Part of the issue is that Vim has built-in ways to convert the selected
range to a single copyable text, while managing complexities of dealing
with blockwise/linewise modes. Previous implementation in MacVim was
lazy in that it just invoked those code, but the side effect was that it
would also copy the text to the system clipboard and pollute Vim's star
register as well. This was quite undesirable because the user has not
done anything other than opening a menu and wouldn't have expected the
system clipboard or the Vim registers to have changed.

In this fix, we unfortunately had to do a little bit more copy-pasting
to extract the useful bits so we can copy the text to a register (but
not system clipboard), invoke the code to convert the register to
clipboard-happy text, restore the register, and then move on. A little
annoying but better than having the unintuitive / annoying behaveior
from before, and this way we don't have to do too much intrusive
refactoring of upstream Vim code as well.

I just missed the fact that getreg() also works and I don't need to hack
to use clip_convert_selection() and the mod on it. This is kind of...
odd, because clip_convert_selection() is essentially doing the same
thing as getreg() with *very* minor differences. You would hope that
would be consolidated and refactored into a single function, but of
course not.

Also cleaned up some comments to make it clear that evaluateExpression
probably would have worked except for the autocmd blocking part.