fixed level sketch copy/paste via clipboard on Windows (using CR+LF)

fixed level sketch copy/paste via clipboard on Windows (using CR+LF)

Before, level sketch copy/paste using Ctrl-c/v in level editor did not
work on Windows, where lines of text in the clipboard are terminated
with CR+LF (carriage return and line feed) instead of simple newlines.

This fixes this bug, and makes parsing level sketches from clipboard
a little bit more robust regarding empty lines and garbage text.