Centralize History feature at board level

[rtxm]

Objective

History recording is currently spread across almost all the significant components (board, column, card, checklist, comment and gallery). This is a bad design in the perspective of making History a pluggable feature.

The objective of this first step is to centralize all History operations in the board component.

Implementation

All components below a board will "bubble" their important actions through calls to answer(event) where event is a tuple (action, source, data):

• action: (str) an action code;
• source is a list of business objects, each component relaying the event up must append itself to it; (asking the community: is using a list here safe? should we use a tuple instead?)
• data is data specific to the action.

In general, when a component receives an event from its children that it is not able to handle itself, it must answer() it up, appending itself to the source by the way.

The board will then record all these actions in the history.

[rtxm]

I'm on this.