Add tween_subtween method for nesting Tweens

No actual functionality yet

Actual subtween functionality implemented

Added documentation for Tween.tween_subtween and SubtweenTweener

Implemented some additional functions

`set_ease`, `set_trans`, and `set_delay`
Documentation only for `set_delay` so far, since I have tested it

Removed set_ease and set_trans

Upon further investigation, the way they are implemented for Tween doesn't appear to work here

Fixed indentation in documentation

Reset subtween when parent loops

Fix return type of `SubtweenTweener.set_delay`

Add notes to documentation

Apply suggestions from code review

Co-authored-by: Tomasz Chabora <kobewi4e@gmail.com>

Apply some suggested changes

- Remove excessive documentation
- Add Tween constructor that takes in SceneTree
- Make `SubtweenTweener::subtween` public so that `Tween` doesn't have to be a friend class

Remove unneeded friend class SceneTree

Remove superfluous documentation describing subtween behavior

Apply suggestions from code review

Co-authored-by: Tomasz Chabora <kobewi4e@gmail.com>

Apply suggestions from code review

Co-authored-by: Thaddeus Crews <repiteo@outlook.com>

Apply suggestions from code review

Co-authored-by: A Thousand Ships <96648715+AThousandShips@users.noreply.github.com>

Early return from `tween_subtween` if the subtween is `null`

scene/animation/tween.cpp

@@ -154,6 +154,25 @@ Ref<MethodTweener> Tween::tween_method(const Callable &p_callback, const Variant
 	return tweener;
 }
 
+Ref<SubtweenTweener> Tween::tween_subtween(const Ref<Tween> &p_subtween) {
+	CHECK_VALID();
+
+	// Ensure that the subtween being added is not null.
+	ERR_FAIL_COND_V(p_subtween.is_null(), nullptr);
+
+	Ref<SubtweenTweener> tweener;
+	tweener.instantiate(p_subtween);
+
+	// Remove the tween from its parent tree, if it has one.
+	// If the user created this tween without a parent tree attached,
+	// then this step isn't necessary.
+	if (tweener->subtween->parent_tree != nullptr) {
+		tweener->subtween->parent_tree->remove_tween(tweener->subtween);
+	}
+	append(tweener);
+	return tweener;
+}
+
 void Tween::append(Ref<Tweener> p_tweener) {
 	p_tweener->set_tween(this);
 
@@ -447,6 +466,7 @@ void Tween::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("tween_interval", "time"), &Tween::tween_interval);
 	ClassDB::bind_method(D_METHOD("tween_callback", "callback"), &Tween::tween_callback);
 	ClassDB::bind_method(D_METHOD("tween_method", "method", "from", "to", "duration"), &Tween::tween_method);
+	ClassDB::bind_method(D_METHOD("tween_subtween", "subtween"), &Tween::tween_subtween);
 
 	ClassDB::bind_method(D_METHOD("custom_step", "delta"), &Tween::custom_step);
 	ClassDB::bind_method(D_METHOD("stop"), &Tween::stop);
@@ -512,6 +532,11 @@ Tween::Tween(bool p_valid) {
 	valid = p_valid;
 }
 
+Tween::Tween(SceneTree *p_parent_tree) {
+	parent_tree = p_parent_tree;
+	valid = true;
+}
+
 Ref<PropertyTweener> PropertyTweener::from(const Variant &p_value) {
 	Ref<Tween> tween = _get_tween();
 	ERR_FAIL_COND_V(tween.is_null(), nullptr);
@@ -854,3 +879,51 @@ MethodTweener::MethodTweener(const Callable &p_callback, const Variant &p_from,
 MethodTweener::MethodTweener() {
 	ERR_FAIL_MSG("MethodTweener can't be created directly. Use the tween_method() method in Tween.");
 }
+
+void SubtweenTweener::start() {
+	elapsed_time = 0;
+	finished = false;
+
+	// Reset the subtween.
+	subtween->stop();
+	subtween->play();
+}
+
+bool SubtweenTweener::step(double &r_delta) {
+	if (finished) {
+		return false;
+	}
+
+	elapsed_time += r_delta;
+
+	if (elapsed_time < delay) {
+		r_delta = 0;
+		return true;
+	}
+
+	if (!subtween->step(r_delta)) {
+		r_delta = elapsed_time - delay - subtween->get_total_time();
+		_finish();
+		return false;
+	}
+
+	r_delta = 0;
+	return true;
+}
+
+Ref<SubtweenTweener> SubtweenTweener::set_delay(double p_delay) {
+	delay = p_delay;
+	return this;
+}
+
+void SubtweenTweener::_bind_methods() {
+	ClassDB::bind_method(D_METHOD("set_delay", "delay"), &SubtweenTweener::set_delay);
+}
+
+SubtweenTweener::SubtweenTweener(const Ref<Tween> &p_subtween) {
+	subtween = p_subtween;
+}
+
+SubtweenTweener::SubtweenTweener() {
+	ERR_FAIL_MSG("SubtweenTweener can't be created directly. Use the tween_subtween() method in Tween.");
+}