sugar-base: add sugar.dispatch.DbusSignal

[sascha_silbe]

This patch adds the classes sugar.dispatch.RelayedSignal and sugar.dispatch.DbusSignal.

RelayedSignal is a base class for distributing signals from some external sources. The external source will usually only be attached if there are actually (internal) listeners to prevent unnecessary overhead.

DbusSignal (deriving from RelayedSignal) relays DBus signals which can be passed either via full name specification (looked up and connected to only on demand) or as an interface instance (in case the caller already got one).

It is intended to provide data store listeners in sugar.datastore.datastore that only listen to the DBus signals if there's actually someone listening for it, thus reducing overhead.

[sascha_silbe]

add classes sugar.dispatch.RelayedSignal and sugar.dispatch.DbusSignal

[walter]

Seems like a win, but pushing to 0.90 as it is late in the release cycle.

[tomeu]

Two comments:

- can you add a link to the patch that makes use of this class?

- if we don't have still a use for RelayedSignal I think it would be better to not have it and refactor later. In Sugar we have been favouring simpler code and refactoring when needed.

[sascha_silbe]

Replying to tomeu:

- can you add a link to the patch that makes use of this class?

See #1681.