Deskbar mode

[Andrzej]

Hi,

I've started experimenting with the deskbar mode (or vertical dock mode 
or...) we discussed previously.

The initial code is at:
https://github.com/andrzej-r/xfce4-panel

Currently done:

- added a "deskbar-mode" property to the panel and plugin classes
   Effective only in vertical panel mode. It arranges plugins horizontally.

- added a "nrows" property  to the panel and plugin classes
   This property denotes a preferred number of rows (in horizontal mode) 
or columns (in vertical mode). Later I'm planning to make sizes of some 
plugins (button-like applets) and arrange them in the rows.
   "nrows" was added specifically for wide deskbars, which currently 
makes some plugins (e.g. launcher) very big, but I think it might be 
useful in normal horizontal and vertical modes as well.

- changed tasklist applet (as a proof-of-concept) to use these 
properties instead of its internal "rotate-vertically" and 
"max-button-size". The old properties are still there, just are no 
longer effective.

Planned:

- add a plugin property "small" (defaults to FALSE).
   If active, that will be a request to the itembar that this plugin 
fits in a single row (as opposed to the full panel width).

- change the plugin layout methods in the itembar.

- change other plugins in a similar way the tasklist was modified.

Could you please let me know your opinion on these changes. In particular:

1. Are you happy having this functionality in your code.
2. Is API and GUI OK?
3. Is the implementation OK (I'm new to XFCE and GTK hacking).

I'll be rather busy next week but if you like this direction I'll 
continue my work afterwards and modify the code according to your comments.

Andrzej