Preventing splitting string after backslash.

I fixed the behavior of splitting string.
Now when we press Enter key, if we're after a backslash, creating new string will be prevented, so we obtain backslash-newline-separated (\\\n in c-style) string.

So, when we typed something like this:
"some\str"
… and hit RET after \ char, we obtain:
"some\
str"
… instead of default behavior:
"some\"
  + "str"
… which is illegal…

Of course, my fix is safe. So, using backslashes for escaping is covered.
Even you typed something like:
"some\\str"
… and hit RET after second slash, you will have
"some\\"
  + "str"
… but if you type third slash after second and hit RET, you obtain
"some\\\
str".