Add Mouse.up & Mouse.down

[jinjor]

I am trying to render selection rectangle that starts by mousedown and ends by mouseup. The start position should be memorized when mouse state changes to isDown == True.

Currently we have to filter Mouse.isDown. I think better to provide two more functions below

Mouse.down : Signal ()
Mouse.up : Signal ()

and Mouse.isDown can be made by their combination.

isDown = merge (always True <~ down) (always False <~ up)

Does this implementation cause any trouble?

Thanks.

[evancz]

Seems plausible, adding to #322 for future consideration in a broader redesign of core.

[jinjor]

Thanks :)

[evancz]

No problem, thanks for raising the idea!