Fix compiler warnings from --enable-debug=yes

Fixed old-style function definitions, dropped unused variables, renamed
signal to xtm_signal (it shadowed a previous global declaration from
signal.h,) dropped the G_UN/LIKELY calls, missing includes and casted
here and there.

The task-manager-*.c files /would/ have to be individually fixed.

src/process-statusbar.c

@@ -153,7 +153,7 @@ xtm_process_statusbar_set_property (GObject *object, guint property_id, const GV
 
 
 GtkWidget *
-xtm_process_statusbar_new ()
+xtm_process_statusbar_new (void)
 {
 	return g_object_new (XTM_TYPE_PROCESS_STATUSBAR, NULL);
 }