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.

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.

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.

This is implemented in xfconf now, so we don't need it
anymore.

This requires a patch for xfconf that disconnects from
DBus using dispose.

The binding will use the default panel and take care of ref-
counting.

If saving is enabled, the bindings won't restore the value
found in the channel, but instead notify to property on
the object so it gets stored.

The shared table is set during panel startup, the panel will load
all the properties and all (internal) plugin will use this table
during startup, which gives a nice performance boost.

The goals of these bindings is speeding up the panel startup.
The bindings can optionally take a hash table for searching
properties, this way the panel only has to call dbus once
during startup for panel settings.

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

Add provider API for getting and showing the configure and about
dialog from the panel. External plugin implementation is not
finished yet.