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/cpu_particles_editor_plugin.cpp

@@ -29,6 +29,7 @@
 /*************************************************************************/
 
 #include "cpu_particles_editor_plugin.h"
+
 #include "editor/plugins/spatial_editor_plugin.h"
 
 void CPUParticlesEditor::_node_removed(Node *p_node) {