Move ParticlesMaterial code to its own resource file

It's shared by both 2D and 3D particles (+ CPU ones), so it makes sense
to have as a common resource. It also allowed to disable compilation of
Particles (3D) when using 'disable_3d'.

Also cleaned up includes in SpatialEditorGizmos and some other places,
as well as dropped dead code in material_editor_plugin.cpp.

editor/plugins/particles_editor_plugin.h

@@ -43,6 +43,7 @@
 class ParticlesEditorBase : public Control {
 
 	GDCLASS(ParticlesEditorBase, Control)
+
 protected:
 	Spatial *base_node;
 	Panel *panel;