Feature/elbow content

Added content to horizontal elbows. This adds the concept of stack panels which are either vertical or horizontal:

 - lcars-stack-vertical
  - Will stack buttons vertically. This is the same behaviour we had for *all* buttons before
 - lcars-stack-horizontal-left
 - lcars-stack-horizontal-right
  - These two will stack horizontally, with elements floating to the left or right

Making an elbow into a stack panel allows buttons inside it to act properly