[$5] Add shortcut for 1:1 zoom

Currently there is no way to set the zoom to 1:1, especially when the image being viewed is larger than the viewport. One can use the slider bar or the +/- keys to try to do a 1:1 zoom, but it's fiddly and inaccurate.

gthumb, for example, uses the 1 key to achieve this. Ristretto uses ctrl+0, and gimp uses ctrl+1