make proper setup for VRPN with buttons and trackers

addons/vrpn/scripts/VRPN_Button.gd

@@ -0,0 +1,18 @@
+class_name VRPN_Button
+extends Node
+
+@export var vrpn : VRPN = null
+@export var button_sensor : String = "Button0"
+@export var register_on_start : bool = true
+
+var state : int = 0
+
+func _ready():
+	if not vrpn:
+		push_warning("No VRPN client for button on '%s' given." % [self.name])
+	elif register_on_start:
+		vrpn.buttons.append(self)
+
+func _on_vrpn(vrpn_data : Dictionary) -> void:
+	if vrpn_data['sensor'] == button_sensor:
+		pass