iv pan & zoom controls should be standard

Jeremy says: Pan and zoom should work the same as every other app. Middle mouse pans. Right mouse zooms. The 'bottom-left to top-right' diagonal should determine the zoom amount, such that moving the mouse either to the right or down zooms. Middle mouse scroll wheel should zoom, and lock to the nearest integer zoom size. +/- on keyboard should also step to next zoom size. (AND also map "=" to "+", so you dont have to hold down shift).

LG says: double check other important image apps to make sure that these assumptions are nearly universal before changing any current behaviors.

No files or tests are named. Start by locating the iv viewer's current mouse and keyboard input handling, then compare the stated pan and zoom conventions with other important image apps; done means the agreed controls and zoom stepping work consistently, with appropriate verification.

Written by the indexing model from the issue text.