Support "run at startup" bindings

It might be useful to allow once-off bindings as an alternative to Component.onCompleted.

For example, rather than determining initial position by:

Component.onCompleted: item.x = item.parent.width/3

You could potentially do something like:

x: Qt.compute(parent.width/3) // or computeOnce, runAtStartup, etc

I think this has the advantage of keeping the code "in-place" for the developer