Make Drag and Drop an application-wide operation.
This allows do drop on Controls in other Viewports/Windows.
In order to achieve this, `Viewport::_update_mouse_over` is adjusted to
remember the Control, that the mouse is over (possibly within nested
viewports). This Control is used as a basis for the Drop-operation, which
replaces the previous algorithm, which was only aware of the topmost
Viewport.
Also now all nodes in the SceneTree are notified about the Drag and Drop
operation, with the exception of SubViewports that are not children of
SubViewportContainers.
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