[texworks] New PDF previewer code

[Philip Taylor]

Also one really needs a "fit spread to screen" view as well as a "fit
page to screen" view; I had originally used the previewer maximised,
where "fit page to screen" also allows "fit spread to screen", but once
the previewer is normalised to one-half screen, "fit page to screen"
prevents much of the spread from being visible, and no other option
gives better rendering of the spread.

** Phil.