lua: make register_event() not overwrite previous event handler

Instead, chain them.

Note that there's no logic to prevent the other event handlers to be run
from an event handler (like it's popular in GUI toolkits), because I
think that's not very useful for this purpose.

DOCS/man/en/lua.rst

@@ -100,6 +100,10 @@ The ``mp`` module is preloaded, although it can be loaded manually with
     associated, the ``error`` field is set to a string describing the error,
     on success it's not set.
 
+    If multiple functions are registered for the same event, they are run in
+    registration order, which the first registered function running before all
+    the other ones.
+
     Returns true if such an event exists, false otherwise.
 
     See `Events`_ and `List of events`_ for details.

player/lua/defaults.lua

@@ -135,7 +135,12 @@ mp.keep_running = true
 local event_handlers = {}
 
 function mp.register_event(name, cb)
-    event_handlers[name] = cb
+    local list = event_handlers[name]
+    if not list then
+        list = {}
+        event_handlers[name] = list
+    end
+    list[#list + 1] = cb
     return mp.request_event(name, true)
 end
 
@@ -180,9 +185,11 @@ _G.mp_event_loop = function()
         mp.suspend()
         more_events = (e.event ~= "none")
         if more_events then
-            local handler = event_handlers[e.event]
-            if handler then
-                handler(e)
+            local handlers = event_handlers[e.event]
+            if handlers then
+                for _, handler in ipairs(handlers) do
+                    handler(e)
+                end
             end
         end
     end