Streamline position property of InputEvent classes

[Sauermann]

Currently the position property of input events is defined in four different classes:

• InputEventScreenDrag
• InputEventScreenTouch
• InputEventGesture
• InputEventMouse

This can be streamlined, so that position is defined only in a single class and here is a proposal of how to do this:

• InputEventFromWindow
  • InputEventWithModifiers
    • InputEventKey
    • InputEventPosition (new)
      • InputEventScreenDrag
      • InputEventScreenTouch
      • InputEventGesture
      • InputEventMouse

Besides the intended change, this would add Modifiers (shift, ctrl, ...) to InputEventScreenDrag and InputEventScreenTouch. This would extend the functionality while simplifying the position-code.

[aaronfranke]

Do you have a practical use case in mind for this change? Is there any use case for doing if input_event is InputEventPosition: for example?

[Sauermann]

@aaronfranke In order to handle InputEvent processing in SubViewportContainer correctly, it is necessary to distinguish between events without and events with positions. My arguments for this distinction can be seen in the "Events without position parameter" section of my comment godotengine/godot#55300 (comment).

This proposal is a functionality, that would improve my PR godotengine/godot#58334

The current best way I found to do this is

if (p_event->has_method("set_position")){ ...

but this would depend on the function being actually exposed.

A different way, would be to check all four different classes, but this would be error prone, as soon as additional events with positions would be added.