Remove animation loop from ParticlesMaterial and move it to
SpatialMaterial for 3D particles and Particles2D for the 2D case.
Added animation to CPUParticles2D as well as the "Convert to
CPUParticles2D" to the PAarticles2D menu.
This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.
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.
