Style unit helpers

[raquo]

Having to do this is annoying, and causes the overhead of another signal:

div(left <-- $int.map(v => s"${v}px"))

We could offer something like this instead:

left.px <-- $int
left.em <-- $double
left.percent <-- $intOrDouble

See what's the best way to do this, maybe something like StyleCodec in SDT?

Also: scale() for transform

Please comment with other helpers that might make sense to do together with this.