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/curve_editor_plugin.h

@@ -35,8 +35,6 @@
 #include "editor/editor_resource_preview.h"
 #include "scene/resources/curve.h"
 
-class EditorNode;
-
 // Edits a y(x) curve
 class CurveEditor : public Control {
 	GDCLASS(CurveEditor, Control);