Always try to set the rgba colormap so we don't need to
restart plugins when compositing changes.

Split the base of the external plugin code in three pieces:
PanelPluginExternal as abstract object to handle the child
monitoring and access-point for the panel and
PanelPluginExternal{46,Wrapper} to handle the dbus or socket
communication. This drop a lot of duplicated code.

Also improve handling of external plugins during a child
crash; we now wait until the child is terminated before
we launch the new proccess and also handle restarts of a
child (requested by the panel) without bothering the
user.

Still not there yet, but this makes things a bit easier
to understand.

In 4.6 the quit action also showed the logout dialog, instead of
quiting the panel. Also drop the restart option since it is not
suitable for normal users (xfce4-panel -r still exists).
This makes it harder to accidentally remove the panel from the
session, if you want that, run xfce4-panel -q (normal quit).

This makes it a lot easier for users to start the panel
after it is somehow lost from the session, just click the
panel preferences button in the settings manager.

This allows to make a plugin unique for a screen. Also
improve (and fix) the handling of unique plugins in the
module code and items dialog.

Add support for colored and image backgrounds. This makes theming the
panel a lot easier and allows us to handle the colors and image
positioning in the external plugins.

The background image does not apply to the autohide window.

Force a quit of an unrealized plugin, so it can be properly
restarted when realized again. Also add some debugging
information for this.

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.