Don't add extra newlines when pasting from GTK applications

Due to some internal handling in GTK itself, we end up with text copied
from Firefox where a newline is represented as CRLF (\r\n); since the
doPaste() method replaces each \n with \r, we ended up with \r\r, which
ultimately led to extra newlines in the pasted code. To fix the issue
simply detect the CRLF case and replace each \r\n with \r.

To test, copy some multiline text from Firefox, open vim in konsole and
paste, you end up with two newlines between each block of text. Compare the
results after applying this patch

BUG: 421480
FIXED-IN: 20.04.3
(cherry picked from commit a1022442)