fix lpp/window-containing-preview

[Simon-Lin]

This is to fix a bug where the latex-preview-pane sometimes fail to find the correct window containing preview when there is more than 3 windows (across multiple frames). The cause of this bug is that the length of windows on line 67 is dynamical so the function misses the body of list. I have re-implemented the function using cl-find. Hopefully this makes the code more readable while maintaining the same functionality.