Inherit Buildable.

* gtk/src/buildable.[hg|ccg]: Added this interface.
* gtk/src/action.[hg|ccg]:
* gtk/src/actiongroup.[hg|ccg]:
* gtk/src/liststore.[hg|ccg]:
* gtk/src/sizegroup.[hg|ccg]:
* gtk/src/treestore.[hg|ccg]:
* gtk/src/widget.[hg|ccg]: Inherit and Implement Buildable.