api: Add id parameter for MainLoop files

This makes it easier to use MainLoop with libraries that may want to
setup the same file descriptor several times (with different event
masks).
dbus: New plugin providing a D-Bus interface

This plugin will allow Licq to be remotely controlled from other
applications via D-Bus. It also emits signals for events allowing osd
style notification applications to be created without building new Licq
plugins.

The current API is limited to just a few simple operations but should
be easy to extend depending on user needs.

The D-Bus access is separated from the Licq plugin logic to make it
reuseable. (For example to make a protocol plugin for the Skype API.)