Panel widgets library

[Andrzej]

I've been thinking about what we can do to improve quality and 
portability of panel plugins (which isn't exactly our strongest point).

Here is one idea:

- split libxfce4panel into two libraries:
   1. libxfce4panel, containing only stuff necessary for talking to the 
panel,
   2. libxfce4panel-widgets (pick your name) - containing additional 
panel-related stuff (panel image, hvbox, new widgets[1]) and is portable 
across panel versions.

Reasons:

- Non-core plugins are expected to be portable across Xfce versions. But 
because they depend on libxfce4panel, it is very difficult to add new 
functions this library - any new features have to be used conditionally, 
which requires changing all the plugins.
- libxfce4panel-widgets could evolve independently from the panel and be 
backward compatible with previous panel/Xfce versions.
- (why not simply use libxfce4ui) the new library would depend on 
libxfce4panel.

[1] we could for example provide a basic plugin widget, which could be 
inserted in the plugin and handle all the resizing, orientation, HIG 
stuff. Currently plugins are rather buggy and inconsistent in this area. 
Plugins that do this stuff right, reimplement the same functionality 
over and over.

Andrzej