Reorganize some code to have 2D things grouped together

[aaronfranke]

This PR takes pieces out of #47054, in the same vain as how #51474 took pieces out of #50147.

• In CanvasItemEditorPlugin, move skeleton_menu declaration next to the rest of the skeleton code.
• In CanvasItemEditorPlugin, put Control-derived options before Node2D-derived ones.
• In SpriteFramesEditorPlugin, move the checks right after the casts they're using.
• In World2D, move get_direct_space_state method up.
• In register_scene_types.cpp, organize the 2D includes, organize 2D classes to be together, and move AudioStreamPlayer3D together with the 3D classes.
• In register_server_types.cpp, organize 2D and 3D navigation code.
• In main.cpp, organize includes and singletons.

[mhilbrunner]

PR review meeting: further internal cleanup may be nice, but not priority for now in 4.0 Beta phase.

[akien-mga]

Looks fine overall.

[AThousandShips]

Looks good to me

[akien-mga]

Thanks!