Changed buttons to require that they are a descendent of .lcars-stack-vertical or .lcars-stack-horizontal-(left|right), this means the buttons know how to place themselves in their parent.
Changed buttons to require that content is inside a span element (maybe this should be any element), this allows the span to be sized properly so that text can be positioned vertically within parent.

Coming soon: spacers in columns which fill any remaining width/height with solid colour.

Signed-off-by: Martin Evans <martindevans@gmail.com>