Add Stopwatch node

[Xrayez]

Complements Timer node in Godot. Allows to measure time intervals.

Resolves godotengine/godot-proposals#2878.

When comparing Timer to Stopwatch:

1. Does not have wait_time nor time_left properties, but time_elapsed.
2. Does not have one_shot, does not make sense in Stopwatch.
3. Does not have paused property. Intervals are measured via Stopwatch.start() and Stopwatch.stop(), yet Stopwatch.stop() does not reset Stopwatch.time_elapsed to zero. You have to use Stopwatch.reset() for this. This is because you can measure multiple intervals in succession. In essence, Stopwatch.stop() freezes the cumulative elapsed time value.
4. Does not have timeout() signal, but interval_measured() (emitted when stop() is called). Can be useful to insert speedrun intervals to a scoreboard, even without having to reset the stopwatch (you can keep track of start and stop times provided by the signal parameters as well).

Example

extends Node

func _process(_delta):
	$Debug.text = str($Stopwatch.time_elapsed)

func _input(event):
	if event.is_action_pressed("ui_accept"):
		if $Stopwatch.is_stopped():
			$Stopwatch.start()
		else:
			$Stopwatch.stop()
	if event.is_action_pressed("ui_cancel"):
		if $Stopwatch.is_stopped():
			$Stopwatch.reset()

func _on_stopwatch_interval_measured(time_interval, time_start, time_stop):
	print("Interval: %s, start: %s, stop: %s" % [time_interval, time_start, time_stop])