Add `GridObjectCollection` from MRTK2 for non-canvas UI layout

[Schroedingers-Cat]

Problem

MRTK2 has a GridObjectCollection which is missing in MRTK3. Here's a link to the original docs.

Solution

Port missing functionality from MRTK2 to MRTK3

Considered Alternatives

Port it myself. I have no idea what's up ahead. Any specifics you'd want to see in a possible PR?

Additional context

I'm porting my project from MRTK2 to MRTK3 and it makes use of the GridObjectCollection several times.

[anonymous2585]

If you are using canvas, you can use Unity's components "Vertical Layout Group", "Vertical Layout Group" or "Grid Layout Group".
The button prefab for canvas is in Packages/org.mixedrealitytoolkit.uxcomponents/Button/Prefabs/Action Button.prefab

If you don't use canvas, why not using them?

[Schroedingers-Cat]

Thanks for the heads up! I hadn't thought of the availability of the layout group components when using the canvas based UX components. This turned out to be a perfectly fine replacement for our use case of the GridObjectCollection script.

For the MRTK2 to MRTK3 migration docs, I'd suggest to just mention the GridObjectCollection component somewhere so that people looking for a replacement can quickly find that info.