Composer zoom improvements

These two commits switch the behaviour of some composer zoom functions from an immediate action (zoom in/out on centre of view) to a mouse-based action (zoom in/out via mouse click, or zoom in on a specific composer region via mouse click-and-drag). The Ctrl+/Ctrl- and menu items for zoom in/out use the immediate action, while the toolbar zoom controls use the mouse based action. 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+alt+space (zoom out). To me, this behaviour is a good balance of matching how other DTP packages operate while also mostly respecting how the qgis map canvas handles zoom operations.