Allow adjusting size of split pane

[michael]

This means we introduce a separator area that is draggable. Challenge will be to make this least graphically obtrusive. However there needs to be some space reserved for the handle (like 10px). This must not overlap with other content.

New option draggable is false by default. Needs to be set specifically.

$$(SplitPane, {
  sizeA: '30%'
  splitType: 'horizontal',
  draggable: true
})

See React SplitPane for a reference implementation:

https://github.com/tomkp/react-split-pane

[khlbrg]

Nice feature!

[obuchtala]

There is an incomplete PR which we will try to pick up in beta-5