Split component layout function

[io7m]

The current layout() function declared on the SyComponentType does the necessary grunt work to lay out the current component, but it then goes on to call layout() on the child components. While it should do this, some components that override this method are only interested in the first half of the work that the method does. Unfortunately, they only have the option to call the entire method and possibly end up laying out child components twice.

Three methods should be provided: layoutThis, layoutChildren and layout. layout calls layoutThis and then layoutChildren. Overrides of layout can call layoutThis and layoutChildren as necessary.