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.

Add a parser to import an xml channel file into a
channel and add a default configuration.

The migration utility will now prompt with a
question dialog whether to import the olf 4.6
config or use the default setup.

Some settings cannot me migrated because they do not
exists anymore, or the setting is not implemented yet.

This application will carefully migrate the old panel
configuration to xfconf. Plugins and their setting are
not ready yet.