Saves 128 bytes in testing with `mftest mega 1 -y`

- Send notifications to ExtUI for M0/M1

- wait_for_user can be non-volatile (not changed by interrupt)
  C / C++ compilers don't optimize away reads of non-volatile variables when a function call is used between accesses, because *any* variable could be changed by the function call. Since `wait_for_user` can't be changed without a function call, it should be non-volatile so the compiler can optimize away cases where it is read more than once without an intervening function call.

* Encapsulate common LCD code in a singleton
* Depend more UBL code on UBL_DEVEL_DEBUGGING
  - Since most users don't need the debugging on at all times, this helps reduce the default build size for UBL by over 2K, a little closer to fitting on 128K boards.

Co-Authored-By: Giuliano Zaro <gmagician@users.noreply.github.com>

Co-Authored-By: ejtagle <ejtagle@hotmail.com>

* Remove obsolete strings
* Tweak some static consts
* Reset LCD status to fallbacks

See https://github.com/MarlinFirmware/Marlin/issues/10246#issuecomment-377146936

* Automatically reset stepper timeout in manage_inactivity

Any code that adds moves to the planner can skip resetting the stepper timeout. We can let `idle` / `manage_inactivity` reset the timer whenever it detects any moves in the planner.

* blocks_queued => has_blocks_queued