Update Grass7Utils.py

Related to https://issues.qgis.org/issues/20632
This seems to solve this issue, but I don't really know if this change may be introducing some collateral effect.
It should be revised by someone with good knowledge of this piece of code.

python/plugins/processing/algs/grass7/Grass7Utils.py

@@ -400,6 +400,11 @@ class Grass7Utils:
         # commands again.
         if not grassOutDone and outputCommands:
             command, grassenv = Grass7Utils.prepareGrassExecution(outputCommands)
+            # For MS-Windows, we need to hide the console window.
+            if isWindows():
+                si = subprocess.STARTUPINFO()
+                si.dwFlags |= subprocess.STARTF_USESHOWWINDOW
+                si.wShowWindow = subprocess.SW_HIDE
             with subprocess.Popen(
                     command,
                     shell=False,