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.