Moving image commands to one submap

[Lars Ingebrigtsen]

When the image keymap was introduced, it only had a couple of commands,
so triggering something by accident wasn't a major worry.  But it's now
grown a large number of bindings, so people are hitting these commands
more by accident.

So I think we should move them all under one prefix, to make this less
likely, and also then make the applicable commands use repeat-map.  For
instance, `+ + +' would become `<something> + + +' etc (to increase the
size of an image).

But what should <something> be?  Something with "image" mnemonics and
simple to type, but `C-i' isn't an option (because of confusion with
`TAB').