Move max_children checking into the base Container class, add empty FixedSplit class ready to implement.
Add FixedSplit implementation, and Label base colours (inline text colouring needs to be a future feature).
Move the sample's app name into a header using the new FixedSplit and coloured label functionality.
Switch main application background to black instead of terminal default.
Add a base foreground too so the text doesn't seem to disappear when the default foreground is also black.