Core: Add Callable.create static method for Variant callables

doc/classes/Callable.xml

@@ -4,7 +4,7 @@
 		A built-in type representing a method or a standalone function.
 	</brief_description>
 	<description>
-		[Callable] is a built-in [Variant] type that represents a function. It can either be a method within an [Object] instance, or a standalone function not related to any object, like a lambda function. Like all [Variant] types, it can be stored in variables and passed to other functions. It is most commonly used for signal callbacks.
+		[Callable] is a built-in [Variant] type that represents a function. It can either be a method within an [Object] instance, or a custom callable used for different purposes (see [method is_custom]). Like all [Variant] types, it can be stored in variables and passed to other functions. It is most commonly used for signal callbacks.
 		[b]Example:[/b]
 		[codeblocks]
 		[gdscript]
@@ -46,6 +46,22 @@
 		    # Prints "Attack!", when the button_pressed signal is emitted.
 		    button_pressed.connect(func(): print("Attack!"))
 		[/codeblock]
+		In GDScript, you can access methods and global functions as [Callable]s:
+		[codeblock]
+		tween.tween_callback(node.queue_free)  # Object methods.
+		tween.tween_callback(array.clear)  # Methods of built-in types.
+		tween.tween_callback(print.bind("Test"))  # Global functions.
+		[/codeblock]
+		[b]Note:[/b] [Dictionary] does not support the above due to ambiguity with keys.
+		[codeblock]
+		var dictionary = {"hello": "world"}
+
+		# This will not work, `clear` is treated as a key.
+		tween.tween_callback(dictionary.clear)
+
+		# This will work.
+		tween.tween_callback(Callable.create(dictionary, "clear"))
+		[/codeblock]
 	</description>
 	<tutorials>
 	</tutorials>
@@ -69,6 +85,7 @@
 			<param index="1" name="method" type="StringName" />
 			<description>
 				Creates a new [Callable] for the method named [param method] in the specified [param object].
+				[b]Note:[/b] For methods of built-in [Variant] types, use [method create] instead.
 			</description>
 		</constructor>
 	</constructors>
@@ -120,6 +137,15 @@
 				Calls the method represented by this [Callable]. Unlike [method call], this method expects all arguments to be contained inside the [param arguments] [Array].
 			</description>
 		</method>
+		<method name="create" qualifiers="static">
+			<return type="Callable" />
+			<param index="0" name="variant" type="Variant" />
+			<param index="1" name="method" type="StringName" />
+			<description>
+				Creates a new [Callable] for the method named [param method] in the specified [param variant]. To represent a method of a built-in [Variant] type, a custom callable is used (see [method is_custom]). If [param variant] is [Object], then a standard callable will be created instead.
+				[b]Note:[/b] This method is always necessary for the [Dictionary] type, as property syntax is used to access its entries. You may also use this method when [param variant]'s type is not known in advance (for polymorphism).
+			</description>
+		</method>
 		<method name="get_bound_arguments" qualifiers="const">
 			<return type="Array" />
 			<description>
@@ -160,7 +186,11 @@
 		<method name="is_custom" qualifiers="const">
 			<return type="bool" />
 			<description>
-				Returns [code]true[/code] if this [Callable] is a custom callable. Custom callables are created from [method bind] or [method unbind]. In GDScript, lambda functions are also custom callables.
+				Returns [code]true[/code] if this [Callable] is a custom callable. Custom callables are used:
+				- for binding/unbinding arguments (see [method bind] and [method unbind]);
+				- for representing methods of built-in [Variant] types (see [method create]);
+				- for representing global, lambda, and RPC functions in GDScript;
+				- for other purposes in the core, GDExtension, and C#.
 			</description>
 		</method>
 		<method name="is_null" qualifiers="const">