Lift state up in SplitPane

[stropitek]

We currently have a "semi-controlled" component which I think is an anti-pattern.

We should make SplitPane "controlled", meaning the size which is passed is always the actual size (same for "closed").

The effect which syncs the external configuration's size of the panel and the local state should reside outside of this component.

@hamed-musallam

[stropitek]

The semi-controlled pattern I refer to is the one implemented in #457

[hamed-musallam]

I think most of the time, not all end users need to control the size and closed properties but I agree that maybe it is better to lift the state up.