lua: report key name for "complex" key bindings

This might make certain use cases less of a mess.

DOCS/man/lua.rst

@@ -253,9 +253,10 @@ The ``mp`` module is preloaded, although it can be loaded manually with
             events (as well as key repeat, if enabled), with the first
             argument being a table. This table has an ``event`` entry, which
             is set to one of the strings ``down``, ``repeat``, ``up`` or
-            ``press`` (the latter if key up/down can't be tracked). It further
+            ``press`` (the latter if key up/down can't be tracked). The
-            has an ``is_mouse`` entry, which tells whether the event was caused
+            ``is_mouse`` entry tells whether the event was caused by a mouse
-            by a mouse button.
+            button. The ``key_name`` entry contains the name of they key that
+            triggered this, or ``nil`` if unknown or invoked artificially.
 
     Internally, key bindings are dispatched via the ``script-message-to`` or
     ``script-binding`` input commands and ``mp.register_script_message``.

player/lua/defaults.lua

@@ -54,10 +54,10 @@ local function reserve_binding()
     return "__keybinding" .. tostring(message_id)
 end
 
-local function dispatch_key_binding(name, state)
+local function dispatch_key_binding(name, state, key_name)
     local fn = dispatch_key_bindings[name]
     if fn then
-        fn(name, state)
+        fn(name, state, key_name)
     end
 end
 
@@ -176,10 +176,11 @@ local function add_binding(attrs, key, name, fn, rp)
             ["r"] = "repeat",
             ["p"] = "press",
         }
-        key_cb = function(name, state)
+        key_cb = function(name, state, key_name)
             fn({
                 event = key_states[state:sub(1, 1)] or "unknown",
-                is_mouse = state:sub(2, 2) == "m"
+                is_mouse = state:sub(2, 2) == "m",
+                key = key_name,
             })
         end
         msg_cb = function()