[texworks] New PDF previewer code

[Stefan Löffler]

Hi,

recently, I finally managed to replace the old PDF previewer code with
the new QtPDF module programmed (largely) by Charlie Sharpsteen. This is
a major overhaul that will allow easy implementation of, e.g.,
continuous page scrolling modes.

For now, everything _should_ work as before, with some significant speed
improvements at high magnifications by a new rendering mechanism and
threading support. Builds are available as usual for Windows
(https://drive.google.com/folderview?id=0B5iVT8Q7W44pNDlQVm9uRGpEWHc&tid=0B5iVT8Q7W44pMkNLblFjUzdQUVE)
and Ubuntu (https://launchpad.net/~texworks/+archive/ubuntu/ppa). Note
that these may contain bugs, so use them at your own risk.

I would very much appreciate any testing of and bug reporting for this
large code revision.

Stay tuned for new exciting features ;).

Best,
Stefan