lua: add timer:is_enabled() function

Allows to query if some timer is currently running or was
stopped/killed.

DOCS/man/lua.rst

@@ -350,6 +350,10 @@ The ``mp`` module is preloaded, although it can be loaded manually with
             with ``add_timeout()``), this starts the timer from the beginning,
             using the initially configured timeout.
 
+        ``is_enabled()``
+            Whether the timer is currently enabled or was previously disabled
+            (e.g. by ``stop()`` or ``kill()``).
+
         ``timeout`` (RW)
             This field contains the current timeout period. This value is not
             updated as time progresses. It's only used to calculate when the

player/lua/defaults.lua

@@ -275,6 +275,10 @@ function timer_mt.resume(t)
     end
 end
 
+function timer_mt.is_enabled(t)
+    return timers[t] ~= nil
+end
+
 -- Return the timer that expires next.
 local function get_next_timer()
     local best = nil