viewer.js: Do not process keyboard shortcuts when focused element is a textarea

If focused element is a textarea, viewer should not process keyboard shortcuts (as it does with input and select elements) because while writing on a textarea, if you press k,p,l, or n, the viewer scrolls next/prev page and the letter is not added to the textarea (eg. in my case I had a viewer displaying a product specs in PDF and a "request a quote" form at it's right, the form had a textarea field and users complains that they couldn't write in it, after checking it, I've realized it was this particular issue and fixd it with the change I'm commiting.