Add zoom scales to composer and zoom to 100% action

This adds a new "100% zoom" action, for quickly zooming a composition to actual size. It also adds a new control to the status bar which shows the current zoom level, and allows for entering an exact zoom scale or choosing from predefined scales via a combo box.

Also includes a small enhancement which makes sure that wheel zoom actions result in a predictable zoom step, so that zooming in then out always returns to the exact original zoom level.