Editor: Cleanup some includes dependencies

Removes some unnecessary includes from `editor_node.h`, and instead add
those where they're used.

Removes unnecessary `editor_node.h` includes in various editor classes.

Renames `dynamicfont` to `dynamic_font` in a couple files.

Misc cleanup while jumping through that rabbit hole.

editor/plugins/mesh_instance_3d_editor_plugin.h

@@ -35,8 +35,6 @@
 #include "scene/3d/mesh_instance_3d.h"
 #include "scene/gui/spin_box.h"
 
-class EditorNode;
-
 class MeshInstance3DEditor : public Control {
 	GDCLASS(MeshInstance3DEditor, Control);