Added signal to prevent window closing if USB printer is printing

CURA-5384

plugins/USBPrinting/USBPrinterOutputDevice.py

@@ -435,6 +435,9 @@ class USBPrinterOutputDevice(PrinterOutputDevice):
 
         self._gcode_position += 1
 
+    def getIsPrinting(self)-> bool:
+        return self._is_printing
+
 
 class FirmwareUpdateState(IntEnum):
     idle = 0