[texworks] New PDF previewer code

[Stefan Löffler]

Dear Philip,

thanks for the suggestions. Different page layouts are already in the
code, but not available yet in the user interface (this will come shortly).
The other suggestions (a "back" button for links and a page grid) are
very interesting, but will require a little more coding.

Anyway, for now my primary objective is to get the existing code to work
properly again; once that is accomplished, I will start adding new
features.

HTH
Stefan