Only invoke enabled actions

This patch adds an ‘enabled’ property to every action.  An event will
only invoke enabled actions.  All actions start out enabled and this
patch provides no way to toggle their status via the public API.

src/Luvent.lua

@@ -115,17 +115,22 @@ local function newAction(callable)
     -- we would not be able to use the function itself to find the
     -- action like normal.
     action.id = tostring(callable)
-    action.interval = 0
+
+    -- This boolean indicates whether or not the action is enabled.
+    -- Events will never invoke disable actions.  This flag makes it
+    -- possible to disable an action without removing it.
+    action.enabled = true
 
     -- If we have a non-zero interval then we need to keep track of
-    -- how often we consider this action for execution.  The property
-    -- below contains the time of when we last called this action, and
-    -- when considering whether or not to call it again we subtract
-    -- the current time from this time and see if it is greater to or
-    -- equal than the interval.  When first creating the action we set
-    -- the property to the current time so that we can start counting
-    -- the clock from the moment we created the action (i.e. now) up
-    -- until the first time the interval elapses.
+    -- how often we consider this action for execution.  The
+    -- properties below help track the time of when we last called
+    -- this action, and when considering whether or not to call it
+    -- again we subtract the current time from this time and see if it
+    -- is greater to or equal than the interval.  When first creating
+    -- the action we set the property to the current time so that we
+    -- can start counting the clock from the moment we created the
+    -- action (i.e. now) up until the first time the interval elapses.
+    action.interval = 0
     action.timeOfLastInvocation = os.time()
 
     return setmetatable(action, Luvent.Action)
@@ -254,6 +259,10 @@ end
 -- @return Boolean true if we can invoke this action again at a later
 -- time and false if we cannot (e.g. if it is a dead coroutine).
 local function invokeAction(action, ...)
+    if action.enabled == false then
+        return true
+    end
+
     if type(action.callable) == "thread" then
         coroutine.resume(action.callable, ...)
         if coroutine.status(action.callable) == "dead" then