[Azoth] Initial architectural support for multientries actions.

Actions now carry a functor with them to invoke on entries instead of
connecting to individual slots.

This is all for #1213.
[Azoth] CreateEntriesActions() for #1213.
[Azoth] Support multiple selection for #1213.
[Azoth] Handle multiple selection for #1213.
[Azoth] Don't show multiactions without actors for #1213.
[Azoth] Refactored some more actions to actors for #1213.
[Azoth] Use boost::visitor, prepare for true multiactions for #1213.