idle_timeout: Move timeout handling from toolhead.py to new extras module

Move the "motor_off_timeout" tracking to a new module in the extras/
directory.  This makes it easier to customize the idle timeout
behavior.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

config/example-extras.cfg

@@ -165,6 +165,17 @@
 #   default is 120.
 
 
+# Idle timeout. An idle timeout is automatically enabled - add an
+# explicit idle_timeout config section to change the default settings.
+#[idle_timeout]
+#gcode:
+#   A list of G-Code commands (one per line) to execute on an idle
+#   timeout. The default is to run "M84".
+#timeout: 600
+#   Idle time (in seconds) to wait before running the above G-Code
+#   commands. The default is 600 seconds.
+
+
 # Multi-stepper axes. On a cartesian style printer, the stepper
 # controlling a given axis may have additional config blocks defining
 # steppers that should be stepped in concert with the primary