The allows a fixed length of the panel, which if the new
alternative of the fixed size in the tasklist plugin.
It can be misused if you set the length of the panel to something
smaller then the size the plugins minimally need.

Most plugins do most of the work in the size-changed signal,
so make that the last property that is set on a plugin startup.

Because of the dbus signal we cannot return the reply
of the remote event, so add a method and handle ids to
reply to the panel.
When an implementation of the panel plugin provider returns
a handle, the dbus service waits for the reply before it
continues.

This makes the whole thing more reliable. Switching
compositing on-the-fly for sockets is a bit tricky.

This avoid the actual drop is at another position on the panel.

Finish the API docs and fix some warnings
that prevented to build a dist tarball. Also add some
macros to make sure we contain all the 4.6 API.

Also add some debugging output in the "positioning" domain.