Hyperlinks and preserving position when changing scale

Hi Julian,

I've made some more additions to the pdfListView which are needed for ShareLaTeX. The two main additions are:

1) External and internal hyperlinks can now be followed. Currently only the 'XYZ' type of internal destinations will work, since changing the 'FIT*' types were more complicated to implement, and this is a solution that will work in 90% of cases at the moment. It will be easy to extend the code to support other destination types at a later date.

2) When changing the scale, the PDF now stays anchored at the same vertical position. Again, this isn't fully complete since it doesn't take into account the horizontal position, but it's a solution that improves things in 90% of cases and is easy to extend later.

James