Fixed Infinite Canvas issues and Pan mode in 3D background preview

EndlessGrid was restructure to be placed in DragCanvas, in order to centralized all handling of the event.

![changing of structure](https://f.cloud.github.com/assets/5276243/1481279/a6ca3158-46c5-11e3-94e5-e0776bb4713a.png)

Fixes included:
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1) Infinite Canvas not working in 3D background preview
2) ZoomBorder size is static and will not work for screen larger than 2500 pixel 
3) Pan Mode conflicting with selection (MAGN-464)