Add options for reverting to default zoom

[vertigo]

Add an option to allow a mouse button to be configured to return the picture to the default zoom level. For example, I use the 'Auto image size' setting 'Fit image to window, large only' and if I zoom in on a picture, I'd like to be able to click a mouse button and return to that zoom level (100% for a picture smaller than the window, fit to window for a picture larger than it). Ideally, this would also be implemented into shortcuts so a key or key combo could perform this action.