Add "fit to width" zoom mode

This zoom mode resizes the width of the image to fit, but not the
height. Also, change the zoom keyboard shortcuts to be more mnemonic.
Move all zoom actions to zoom menu

There's now only one zoom button (with an arrow added to indicate it
opens a menu). I think this looks tidier and more organized, and all the
zoom keyboard shortcuts are shown in the UI now.
Initialize more variables

This is probably a good idea.