Input: add KEY_IMAGES specifically for AL Image Browser

Many media center remotes have buttons intended for jumping straight to
one type of media browser or another -- commonly, images/photos/pictures,
audio/music, television, and movies. At present, remotes with an images
or photos or pictures button use any number of different keycodes which
sort of maybe fit. I've seen at least KEY_MEDIA, KEY_CAMERA,
KEY_GRAPHICSEDITOR and KEY_PRESENTATION. None of those seem quite right.
In my mind, KEY_MEDIA should be something more like a media center
application launcher (and I'd like to standardize on that for things
like the windows media center button on the mce remotes). KEY_CAMERA is
used in a lot of webcams, and typically means "take a picture now".
KEY_GRAPHICSEDITOR implies an editor, not a browser. KEY_PRESENTATION
might be the closest fit here, if you think "photo slide show", but it
may well be more intended for "run application in full-screen
presentation mode" or to launch something like magicpoint, I ...