Fix gcc4 warning.
Move B_MOUSE_IDLE generation to app_server.

* BWindow used to generate the B_MOUSE_IDLE events by sending a
  delayed message with a one-shot BMessageRunner to itself.
  Every creation and deletion of BMessageRunners causes synchronous
  messaging between the application under the mouse cursor and the
  registrar. This creates large amounts of calls to set_port_owner()
  in the kernel whenever moving the mouse.
* Now, B_MOUSE_IDLE is sent by the app_server instead. It has a new
  thread "cursor idle loop" which checks for an idle cursor every
  20ms. When the idle time reaches the tooltip delay time, it inserts
  a B_MOUSE_IDLE message into the event stream.
* The tooltip delay thus becomes a system-wide constant and is not
  configurable per-application anymore (no code currently in the
  Haiku repo makes use of that anyhow).