winit key repeating

[csnewman]

In the winit backend key repeating is enabled, whereas when using libinput no such feature exists.

winit backend should have key repeating disabled to be consistent (or atleast an option to disable it).

[elinorbgr]

This needs changes from winit, (https://github.com/rust-windowing/winit/issues/753) so that it tells us which key events are repetitions (so that we can discard them).