Survey (Grid) Zoom with a trackbar

Added:

Ability to zoom with the trackbar. (like in flightplanner or the main MP maps)

Last part of Issue: #563

![zoom](https://cloud.githubusercontent.com/assets/3828006/3885622/a8726fca-21c6-11e4-93ff-d41c18ae1985.png)


I am working on pan with left mouse button, but haven't got it to move smoothly yet.