Add ScrollViewport action

[altskop]

This PR introduces a new Screen Action, ScrollViewport.

Unlike the existing Scroll action, which only offers relative scrolling in one dimension (X or Y), ScrollViewport provides the flexibility to scroll to precise coordinates (which could either be relative or absolute).

For example, in a use case where you have a viewport that has to scroll to the center upon a button press, Scroll action is insufficient. It lacks control over both dimensions and requires the developer to set the scroll amount, which often is not a calculation that can be done without knowing the viewport's current adjustment values.

With the introduction of ScrollViewport, it is as easy as this:

action ScrollViewport("test_viewport", (0.5, 0.5), 0.5)

This action will cause the viewport with the id test_viewport to scroll to its center in 0.5s.

Considerations:

• Scroll action could be further enhanced by allowing providing specific value to scroll to, vs the amount of pixels/pages to scroll by. It would still lack control over both dimensions, but a set of two Scrolls would achieve the same effect as the ScrollViewport action. Let me know if you'd prefer this option instead.

[mal]

My first impression was that this probably belonged in the existing Scroll action, but after seeing how geared towards relative movement that action is I became less convinced.

Either way, I've included a code suggestion to simplify and take advantage of previous changes in the current release cycle.

[mal]

This section can be simplified significantly by leveraging changes merged earlier in the 8,2,0 cycle.

Suggested change

	xadjustment = d.xadjustment
	yadjustment = d.yadjustment
	adjustment = (xadjustment, yadjustment) # tuple of x and y adjustments to use in the loop below
	xyadjustment_change = [] # x and y relative adjustment changes for the animate call based on the position and the current adjustment values
	for i, pos in enumerate(self.position):
	# a loop for x and y
	# using a loop here to avoid code duplication and allow mixing and matching of absolute and relative values
	if isinstance(pos, float) and not isinstance(pos, absolute):
	# If x/y is a float, use it as a relative position
	xyadjustment_change.append(pos * adjustment[i].range - adjustment[i].value)
	else:
	# otherwise, use it as an absolute position
	xyadjustment_change.append(pos - adjustment[i].value)
	if self.delay == 0.0:
	# if there is no delay, call change and sum the x and y adjustment changes to arrive at the final position
	xadjustment.change(xadjustment.value + xyadjustment_change[0])
	yadjustment.change(yadjustment.value + xyadjustment_change[1])
	else:
	# otherwise, animate the x and y adjustment changes
	xadjustment.animate(xyadjustment_change[0], self.delay, _warper.ease)
	yadjustment.animate(xyadjustment_change[1], self.delay, _warper.ease)
	px, py = self.position
	x = absolute.compute_raw(px, d.xadjustment.range)
	y = absolute.compute_raw(py, d.yadjustment.range)
	if self.delay:
	d.xadjustment.animate(x - d.xadjustment.value, self.delay, _warper.ease)
	d.yadjustment.animate(y - d.yadjustment.value, self.delay, _warper.ease)
	else:
	d.xadjustment.change(x)
	d.yadjustment.change(y)