[Rd] Add "Idle" as possible "event" in `dev.capabilities("events")`?

[Trevor Davis]

Dear R Devs,

* `grDevices::dev.capabilities("events")` currently only reports current
graphic device support for the "MouseDown", "MouseMove", "MouseUp", and
"Keybd" "events"
* `grDevices::getGraphicsEvent()` supports functions for those four events
plus an additional "Idle" "event"
* It could be nice if "Idle" was listed as a possible "event" in
`dev.capabilities("events")`
* Currently `x11()` supports the "Idle" "event" on *nix systems but not
Windows systems so developers would need to additionally check the OS on
top of `dev.capabilities("events")` to be sure if the "Idle" "event" was
supported and such a check risks becoming obsolete in the future (if new
interactive graphic devices besides `x11()` supports these events or "Idle"
event support eventually gets added in Windows).

Thanks,

Trevor

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