Attribute marking property as rooting only, or as reactive

[mortoray]

We need a property to indicate changes to a property only take effect at rooting time. This has come up before, and again in https://github.com/fusetools/fuselibs-public/issues/779 It is both necessary so we can show it in the docs, and so that Preview knows it has to recreate that part of the tree if it changes.

Alternately, which might make more sense, is to mark a variable as supporting dynamic changes. These might actually be greater in number. I'm not sure.

@yupferris As our expert on the lower level now is this something that must be done at Uno level (do we need an Uno issue)

We might also consider adding a reactive marker at the same time for properties that properly behave in a reactive system (they can be used in expressions for example).