Retain user-selected presence state, optimize daemons usage

In the course of solving bug 1130084, a couple of wishlist issues came up, as per the attached IRC conversation with Ted:

- we can avoid running the telepathy-indicator, when the presence is set to "offline"
- indicator-messages should actively set the telepathy presence, instead of broadcasting a signal (which can be lost, if no listener is running)

My initial idea to fix these issues was for indicator-messages to scan a certain directory for installed bridges (and telepathy-indicator would be one of them) and invoke D-Bus methods on them as needed (when the user changes the presence state, and maybe also when a message source is clicked). A simple D-Bus interface should be defined by indicator-messages and implemented by the bridges; it would mainly consist of a SetStatus method. D-Bus would then take care of activating the bridges only when necessary (the bridge itself could quit when SetStatus("offline") is called). The current com.canonical.indicator.messages.service interface would remain unchanged, and the StatusChanged signal would continue to be emitted, for the benefit of existing clients which don't need this treatment.

However, Ted pointed out that in fact Telepathy is the only bridge which would be affected by this, and probably the most important consumer of indicator-messages; therefore, it makes sense to take a shortcut and directly let indicator-messages talk to telepathy and set the presence on it.