Feature: arrow keys for controlling current position

[tlambert03]

We are in general need of a keypress framework (which would allow us to also get back to #44) ... but among things that would be nice to have keyboard support for, changing the current slice would be right up there

[gselzer]

Tangentially, do you think it makes any sense for psygnal to absorb some of the keypress/mouse pressing functionality? I wonder whether MouseMoveEvent et. al, MouseButton, even Mouseable could be more reusable if they lived there. Not sure if it makes sense, just an idea.

[tlambert03]

probably not. psygnal currently has no particular awareness of GUI stuff at all. all it assumes is that there is some event loop (which is by no means only in a GUI context). I like the scope there as is