Flymake uses proper idle timers

Flymake uses proper idle timers

Also, flymake-no-changes-timeout can be set to nil to disable
automatic periodic checks.  But even in that situation the idle timer
still runs at a reduced rate to detect changes in the variable and
revert that decision.

* lisp/progmodes/flymake.el (flymake-no-changes-timeout): Improve doc.
(flymake-last-change-time): Delete.
(flymake--schedule-timer-maybe): New helper.
(flymake-after-change-function): Use it.
(flymake-on-timer-event): Delete
(flymake-mode): Don't scheduler timer.