:wrench: Move responsibility for hidding slides

Allow slide preview (if the slide hides itself, you can't see anything
if starting this slide only).

godot/projector/show/Show.gd

@@ -19,6 +19,7 @@ var current_slide: Slide
 
 func _ready():
 	_enforce()
+	hide_every_slide()
 	go_slowly_to_next_slide()
 
 
@@ -46,6 +47,11 @@ func focus_slide(id: int) -> Slide:
 	return slide
 
 
+func hide_every_slide() -> void:
+	for slide in slides:
+		slide.set_visible(false)
+
+
 func _disconnect_current_slide():
 	if current_slide != null:
 		current_slide.remove_focus()

godot/projector/slide/Slide.gd

@@ -11,7 +11,6 @@ var center_offset: Vector2
 
 func _ready():
 	center_offset = _compute_center_offset()
-	set_visible(false)
 	remove_focus()