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.

Each panel can be locked individually though Xfconf and
new API function xfce_panel_plugin_get_locked() for plugins;
although it might be safer for plugins to directly check the
Xfconf channel.

Add a quick signal back to the code and improve the internal
handling so the objects are better isolated.

Bug #5707.