[Feature Request] Split pane

[Arcnor]

Like other windowing toolkits, a split pane is useful to have resizeable areas of the window that hold other widgets, similar to Qt's QSplitter or ImGui splitter behavior.

[djowel]

This is now implemented in the adjustable_grid_skia branch.

Still WIP. The vertical divider will still have to be implemented. But that should come easy following the horizontal divider implementation.

[djowel]

This is now implemented in the adjustable_grid_skia and adjustable_grid branches.