Move texture property back to the top level of the Particles inspector.

[clayjohn]

We recently merged #95132 to re-order properties in the Particle2D inspectors for consistency with their 3D counterparts. In the midst of that, the texture property was also moved (despite not having an equivalent in 3D). Unfortunately, it was moved to be hidden under the new "Process Material" subsection even though it is unrelated.

This change adds confusion in two ways:

1. In order to use the particle at all, it needs a texture, which means that users need to find the texture property in the last submenu which happens to have an unrelated name in order to use particles at all
2. Its different from both GPUParticles3D (which has a mesh property in its own subsection) and CPUParticles2D (which has texture under the "drawing" section). So it moves us even further from being consistent despite that being the stated purpose of GPU/CPU particle parameter list consistency changes #95132

I suggest we bring the texture property back up to the top level and do the same with the CPUParticles2D to keep them consistent.

I suspect that having the process material in a submenu will annoy users as it adds a click for them just to set a property that we know in advance that they have to set. But I haven't seen any complaints, and it does match GPUParticles3D, so I left that alone.

[KoBeWi]

This makes the process_material to be in 1-element group.

[clayjohn]

This makes the process_material to be in 1-element group.

@KoBeWi Yep! That is exactly how it is for GPUParticles3D too. Personally, I would have left the process_material out of a group. I noted my concern with it in the last paragraph of my original description:

I suspect that having the process material in a submenu will annoy users as it adds a click for them just to set a property that we know in advance that they have to set. But I haven't seen any complaints, and it does match GPUParticles3D, so I left that alone.

[paddy-exe]

LGTM and makes sense👍🏻

[akien-mga]

Thanks!