defaults.lua: support scalable mp.add_key_binding()

For complex key bindings, the table now contains a new member
of the current key scale. mp.add_key_binding() now accepts the
scalable flag to make the binding scalable.

DOCS/man/lua.rst

@@ -323,6 +323,13 @@ The ``mp`` module is preloaded, although it can be loaded manually with
             If set to ``true``, enables key repeat for this specific binding.
             This option only makes sense when ``complex`` is not set to ``true``.
 
+        ``scalable``
+            If set to ``true``, enables key scaling for this specific binding.
+            This option only makes sense when ``complex`` is set to ``true``.
+            Note that this has no effect if the key binding is invoked by
+            ``script-binding`` command, where the scalability of the command
+            takes precedence.
+
         ``complex``
             If set to ``true``, then ``fn`` is called on key down, repeat and up
             events, with the first argument being a table. This table has the
@@ -350,6 +357,10 @@ The ``mp`` module is preloaded, although it can be loaded manually with
                     description of ``script-binding`` command for details (this
                     field is equivalent to the 5th argument).
 
+                ``scale``
+                    The scale of the key, such as the ones produced by ``WHEEL_*``
+                    keys. The scale is 1 if the key is nonscalable.
+
     Internally, key bindings are dispatched via the ``script-message-to`` or
     ``script-binding`` input commands and ``mp.register_script_message``.