Restyle the music player bar

The bar is rounded and a little taller, with a round orange play button
and a restart button that lights up orange under the mouse. The icons sit
inside their buttons with room around them, and are one colour, cropped to
their shape. Progress fills in orange, the volume slider gets a speaker
icon, and the buttons say their shortcut keys in their tooltips.

Buttons grow a little under the mouse and shrink while held down, and the
play button pops when it switches between play and pause. ButtonBounce
holds these animations so other buttons can use them too.

common/button_bounce.gd

@@ -0,0 +1,69 @@
+class_name ButtonBounce
+extends RefCounted
+## Small animations that make buttons feel alive: a button grows a little under the mouse,
+## shrinks while it is held down, and can pop when its icon changes.
+##
+## The button is scaled around its middle, so its place in a container does not change.
+
+## How big a button is under the mouse.
+const HOVER_SCALE: Vector2 = Vector2(1.08, 1.08)
+## How big a button is while it is held down.
+const PRESSED_SCALE: Vector2 = Vector2(0.92, 0.92)
+## How small a button starts when it pops.
+const POP_SCALE: Vector2 = Vector2(0.8, 0.8)
+## How long growing or shrinking takes, in seconds.
+const DURATION: float = 0.08
+## How long a pop takes, in seconds.
+const POP_DURATION: float = 0.18
+
+const _TWEEN_META: StringName = &"button_bounce_tween"
+const _PIVOT_META: StringName = &"button_bounce_pivot"
+
+
+## Makes [param button] grow under the mouse and shrink while held down.
+static func add_to(button: BaseButton) -> void:
+	_keep_pivot_centred(button)
+	button.mouse_entered.connect(func() -> void:
+		if !button.disabled:
+			_scale_to(button, HOVER_SCALE))
+	button.mouse_exited.connect(func() -> void: _scale_to(button, Vector2.ONE))
+	button.button_down.connect(func() -> void: _scale_to(button, PRESSED_SCALE))
+	button.button_up.connect(func() -> void: _scale_to(button, resting_scale(button)))
+
+
+## The size [param button] goes back to when it is not held down: bigger under the mouse.
+static func resting_scale(button: BaseButton) -> Vector2:
+	return HOVER_SCALE if button.is_hovered() && !button.disabled else Vector2.ONE
+
+
+## Shrinks [param button] and lets it spring back, e.g. when its icon changes.
+static func pop(button: BaseButton) -> void:
+	_keep_pivot_centred(button)
+	button.scale = POP_SCALE
+	var tween: Tween = _new_tween(button)
+	tween.set_trans(Tween.TRANS_BACK).set_ease(Tween.EASE_OUT)
+	tween.tween_property(button, ^"scale", resting_scale(button), POP_DURATION)
+
+
+static func _scale_to(button: BaseButton, target: Vector2) -> void:
+	var tween: Tween = _new_tween(button)
+	tween.set_trans(Tween.TRANS_QUAD).set_ease(Tween.EASE_OUT)
+	tween.tween_property(button, ^"scale", target, DURATION)
+
+
+# A new tween for [param button], stopping the one before so they never fight
+static func _new_tween(button: BaseButton) -> Tween:
+	if button.has_meta(_TWEEN_META):
+		var running: Tween = button.get_meta(_TWEEN_META)
+		if running.is_valid():
+			running.kill()
+	var tween: Tween = button.create_tween()
+	button.set_meta(_TWEEN_META, tween)
+	return tween
+
+
+static func _keep_pivot_centred(button: BaseButton) -> void:
+	button.pivot_offset = button.size / 2
+	if !button.has_meta(_PIVOT_META):
+		button.set_meta(_PIVOT_META, true)
+		button.resized.connect(func() -> void: button.pivot_offset = button.size / 2)

common/button_bounce.gd.uid

@@ -0,0 +1 @@
+uid://c4vekt3m8q1p7