Keep position when zooming with mouse wheel

Submitted by Allison (desrt)

evince should have an eog-style UI mode where you have these semantics:

scroll wheel zooms in/out (scrolling is sensitive to location -- if you are pointing at a specific region of the document and scroll up then you zoom it to that region). similarly, if you're zooming out then it's done in such a way that the position of the mouse cursor doesn't change with respect to the document.
click+drag moves you around in the document

for certain types of documents, this navigational mode is absolutely amazing (large detailed maps, for example)

Version: git master