SplitPanel - Add arrow keys handling / Not resizing when using mouse

[Orbisman]

Enhancements for the SplitPane:

• Add the ability to use UP / DOWN arrows keys on a 'Horizontal' SplitPanel when it has focus
• Add the ability to use LEFT / RIGHT arrows keys on a 'Vertical' SplitPanel when it has focus.

Standard keys, TAB / <Shift - TAB> work to navigate

Fixes:

• When using mouse, the SplitPanel is not resizing when dragging.
• In a vertical layout, the width of the components are not expanding past their initial sizes