add Function Action
fix
Document self-voicing, generate doc includes.
sl2: Pass default keyword arguments to the displayable.

This is required so we can pass the layout argument to MultiBox,
which makes the distiniction between fixed, horizontal, and vertical
layouts when the style does not.
Remove spurious code.
Merge pull request #384 from kyouryuukunn/Function

Add Function action, which calls a function with arguments.
Allow RevertableDict to be past as the properties argument to Style.

Fixes #323.