Named columns for FullHelp view

[wI2L]

The current implementation of the FullHelp method of the KeyMap interface simply returns a 2d list of bindings.

Grouping bindings in columns make sense, but it would be nice if we could give a name to each column.

To achieve that, I think we could introduce a new type:

type BindingsColumn {
  Name     string
  Bindings []key.Binding
}

that would define the list of bindings and the name of the column (a.k.a binding group).

This implies that we should either modify the prototype of the FullHelp interface method, or introduce a new one.

WDYT?