PySide: [skip ci] add minimal example of embedding

src/Tools/embedded/PySide/minimal.py

@@ -0,0 +1,25 @@
+import sys
+from PySide2 import QtCore, QtGui, QtWidgets
+import FreeCAD, FreeCADGui
+
+class MainWindow(QtWidgets.QMainWindow):
+    def showEvent(self, event):
+        FreeCADGui.showMainWindow()
+        self.setCentralWidget(FreeCADGui.getMainWindow())
+
+app = QtWidgets.QApplication(sys.argv)
+mw = MainWindow()
+mw.resize(1200,800)
+mw.show()
+
+# must be done a few times to update the GUI
+app.processEvents()
+app.processEvents()
+app.processEvents()
+
+import Part
+cube = Part.makeBox(2,2,2)
+# creates a document and a Part feature with the cube
+Part.show(cube)
+app.processEvents()
+app.processEvents()