Composer zoom improvements v2

(This is an improved version of an earlier request -- this version leaves the existing zoom in/out actions untouched, but adds a new "zoom" tool for compositions)

These two commits add a new mouse-based zoom tool to the composer. This tool allows you to to zoom in on a specific composer region via mouse click-and-drag, or zoom out on a specific point via shift-clicking.
The second commit makes it possible to switch to the mouse based zoom in/out at any time by holding ctrl+space (zoom in) or ctrl+shift+space (zoom out). 

This behaviour seems a good trade off between mimicking other standard dtp package functionality while respecting the same behaviour as the main QGIS window.