js: Substantially simplify timer code

The C code in the Emscripten front-end already keeps a timer_active
variable to ensure that the timer can be activated only when it's
inactive, and deactivated only when it's active.  Adjusting the
JavaScript side to rely on this makes the code much simpler.  The only
oddity is that it now requests a new animation frame before calling the
callback so that it's ready to be cancelled if the callback decides to
deactivate the timer.

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