Introduce Viewport functions for keeping the mouse over state consistent

[Sauermann]

• resolve Pushing events to Viewports no longer triggers InputEventMouseMotion in _gui_input #89757
• resolve ScrollBar no highlight in 3D #98003
• resolve parts of Area2D's don't receive input from Subviewports #97267
• resolve duplicate Subviewport's push_input does not do object picking #90413
• supersedes Fix mouse-in-viewport state for Viewport in TextureRect #89868
• supersedes Send mouse-enter notification, when viewport receives a mouse motion event #90444
• usage demo Fix notifying viewport about mouse status in Gui-in-3D-demo godot-demo-projects#1142

Some users had problems with forwarding input events to SubViewport since #88992, which adjusted a default value and in effect prevented calling gui_input when used without changes.
I still believe, that this change makes mouse-handling more consistent. But this change makes it necessary to give users tools and instructions about how to deal with this new situation.
Previously I did believe, that sending NOTIFICATION_VP_MOUSE_ENTER and NOTIFICATION_VP_MOUSE_EXIT was sufficient (see godotengine/godot-demo-projects#1139), but the MRP of #98003 demonstrates, that this approach still leaves some situations unresolved.

So I want to introduce with this PR two functions, that allow users to notify viewports about the mouse-over state. These two functions take care of all internal state-adjustments.

I discarded #90444 as a solution, because while it automatically handles the mouse-enter-part, users still have no solution for the mouse-exit-part.
I discarded #89868 as a solution, because it is restricted to TextureRect and doesn't provide a generic solution for all potential relevant nodes.

The introduction of these two function with their descriptions could be seen as either a partial or a total fix of issue #97267.

[akien-mga]

@eviltrout @SubSage @rakkarage @krdluzni Would some of you be able to test and confirm that this PR would satisfyingly resolve your linked issues?

Note that this requires modifying the project code slightly to use those new methods.

[rakkarage]

thanks. with this pr, adding:

func _ready() -> void:
	_area.input_event.connect(_on_input_event)
	_area.mouse_entered.connect(_viewport.notify_mouse_entered)
	_area.mouse_exited.connect(_viewport.notify_mouse_exited)

fixes the scroll bar not being highlighted while buttons are in 3d.

#98003

[eviltrout]

This does not fix #89757, but over time I have been convinced it's an edge case that most people will not be concerned with, plus there is a workaround that works well. I have closed my issue.

[KoBeWi]

The methods work correctly.

Probably needs better documentation on how to apply them, but it might be out of scope for class reference. Also the official GUI 3D can be updated to call the new methods and adjusted in a way that makes them useful.

[Repiteo]

Thanks!