Add a size method to UpdateCtx and LifeCycleCtx.

[jneem]

I found myself needing this, for a widget that wants to update its internal state based on changes to data, but needs the size to do so (think of something like a scroll bar, whose state depends on the viewport size).

More generally, is there a rationale somewhere for which contexts have which functions? For example, I see that LifeCycleCtx is also missing size...

[luleyleo]

As far as I know, the current rational is "we add it when we need it".

[xStrom]

Yeah I think this is something that's fine to have on LifeCycleCtx as well. Feel free to add it as well, or say if you would prefer to just have this merged.

[jneem]

Ok, I just added it to LifeCycleCtx as well.

[xStrom]

Thanks!