Combine 4 trading actions of the market protocol into 2

[tzhan28]

Market protocol has 4 possible trading actions

buy long token
sell long token
buy short token
sell short token

It could be confusing to new users. It would be more user friendly if the 4 actions can be combined into 2.

Seth, the founder of market protocol argued that only he two buy action should be significant. the sell function should be presented as advanced function.
Jie proposed that it's possible to combined the two in technical terms.