ui,fw: fixed enable/disable global button

When there were no nodes connected, we disabled the global fw button
that enables/disables the fw.

Unfortunately when a node connected to the GUI, this button was not
clickable anymore.

(cherry picked from commit aa0d5f1)

Author: gustavo-iniguez-goya

ui/opensnitch/dialogs/firewall.py

@@ -210,7 +210,11 @@ def _check_fw_status(self):
 
         try:
             enableFw = False
-            if self._nodes.count() == 0:
+            enableFwBtn = (self._nodes.count() > 0)
+            self.sliderFwEnable.blockSignals(True)
+            self.sliderFwEnable.setEnabled(enableFwBtn)
+            self.sliderFwEnable.blockSignals(False)
+            if not enableFwBtn:
                 return
 
             # TODO: handle nodes' firewall properly