convenience code for choosing command and icon in xfce4-panel

[Jasper Huijsmans]

FYI,

I just split out the code from panel/item_dialog.c to choose a command
(using the completion combo if available) and to choose an icon, so it
can be used by others (plugin writers).

Look at item_dialog.h and item_dialog.c to see how it works. The
mailcheck plugin and, obviously, the item dialog already use it.

Danny, could you have a look and tell me if this is anything like what
you had in mind?

	Jasper