Track the Instant at which each UiEvent is created once Rust 1.8 lands

[mitchmindtree]

Looks like the Instant API in std will stabilise in the next stable rust release. I was going to do this using PreciseTime from the time crate in #703, but it looks like we might as well wait a few days and use the std type instead which should make our use of the time crate redundant.

This will be useful for creating time-oriented events such as DoubleClick