Complete rewrite of Tweens

* Tweens were changed from Node to RefCounted. New API is inspired by DOTween.
* Tweens are created and managed by SceneTree, similar to SceneTreeTimer, which makes them ultra cheap to use a lot.
* Animating with Tweens is done by creating sequences of Tweeners. You create them from code and they autostart by default (fire-and-forget).
* There are 4 Tweeners that cover the former Tween functionality: PropertyTweener, IntervalTweener, CallbackTweener and MethodTweener.
* The methods were simplified a lot. Long argument lists are replaced with chained calls on Tweens and Tweeners.
* Tweeners by default execute in sequence, so it's easy to create complex chained animations.
* You can bind a Tween to a node. Tween will be removed automatically when the bound node is freed.

doc/classes/SceneTree.xml

@@ -90,6 +90,13 @@
 				The timer will be automatically freed after its time elapses.
 			</description>
 		</method>
+		<method name="create_tween">
+			<return type="Tween">
+			</return>
+			<description>
+				Creates and returns a new [Tween].
+			</description>
+		</method>
 		<method name="get_first_node_in_group">
 			<return type="Node">
 			</return>
@@ -135,6 +142,13 @@
 				Returns a list of all nodes assigned to the given group.
 			</description>
 		</method>
+		<method name="get_processed_tweens">
+			<return type="Array">
+			</return>
+			<description>
+				Returns an array of currently exising [Tween]s in the [SceneTree] (both running and paused).
+			</description>
+		</method>
 		<method name="get_rpc_sender_id" qualifiers="const">
 			<return type="int">
 			</return>