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.

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.

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.

With this function a plugin can remove itself from the panel
without user intervention.

Move the enums used for this in the provider header.

The panel is the only applciations launching the wrapper,
so use fixed locations for data in the argv.

This will reduce the amount of dbus messages during
startup. Also easy to work with transactions in the
future, but that is not needed right now.

This allows to send messages to plugins using the (hidden)
xfce4-panel command line option
--plugin-event=plugin-name:name:type:value. This can for
example be used to popup a plugin menu from a script.

Plugins running in a wrapper don't support the boolean return
value yet.

We now set a unique path for each wrapper that will
show up under org.xfce.Panel. This makes it easier to
understand in the wrapper (no message filtering) and moves
all the external communication in the panel-plugin-external.c
code.

Not code that is suitable for plugin developers. Also
drop the I_() macro and use g_intern_static_string directly.

Remove unneeded INCLUDES from makefiles.

Move the libexec executables to $(libexecdir)/xfce4/panel/.

Bump the library ABI version to -2, people need to make changes
to the build files for their plugins, so better do it the
right way from the beginning.

Move plugin libraries to $(libdir)/xfce4/panel and desktop
files to $(datadir)/xfce4/panel. Much more consistent.

This signal is only emitted when the user permanently removes
a plugin from the panel and can be used by developers to
removed custom settings. Bug #5694.

Add support to the wrapper to run a pre-init function before
gtk_init() is called. The g_thread_init() function has also
been dropped, developers can call this in pre-init if they
need it.

Hopefully it will make less trouble on the platforms with
support for the prctl call.

This drop quite a bit of code and it highlights the active
panel a bit better on some backgrounds.