Things todo, in any particular order:

 - The button 'Evaluate' actuvates twice the callback 'callback_run_xpath' when
   activated with the spacebar key.

 - Run the XPath expressions in a different thread. This turn out to be a bad
   idea since perl has some special semantics for threads. Also perl doesn't
   handle well the blessing of variables. And for last, Gtk2 doesn't like
   having threads created once the widgets are created.

 - Keep an history of the XPath expressions.

 - Fix a segmentation fault when running XPath expression that are bad but have
   a valid syntax. For example, calling a function that doesn't exist, in such a
   situation the code will create a core dump.
	 Ex: src/gtk-xpath.pl porucik.xml '//switch[stringl-length(default_gw_IP) > 0]'
