Switch Find entry text size to px

Switch Find entry text size to px

All our font sizes were in px, other than the size for the text in
the search entry. Switch that from 12 to 16px (which is the same size
at the standard 96dpi), it doesn't become out of scale if the DPI
is different. (Using fixed px sizes isn't probably what we want to
do in the long term; moving to style sheets will be a good place to
fix that. But better to be consistent.)

http://bugzilla.gnome.org/show_bug.cgi?id=593212