Use some global xyz.Scene variable to enable xyzcore button values to work correctly

[kkoreilly]

Describe the feature

Button values in xyzcore do not have enough context to get the xyz.Scene in which they reside, which is necessary for creating appropriate value selection capabilities. Since there is only ever really one xyz.Scene at a time, we can resolve this by storing a global scene and then just using that for context.

Relevant code

No response