added filter conditions of status such as 'unread', 'mark', 'color-label', and 'mime'.
partially implemented search folder properties.
added a function for minor code cleanup.
implemented query search and virtual folder (merged from newsearch branch).
prefs_filter_edit_cond_hbox_to_cond(): added an argument for case sensitivity.
made more prefs_filter functions public.
prefs_filter_edit.c: generalized the condition edit.
made CondHBox / ActionHBox related APIs public.
Initial import of Sylpheed (GTK2 version).