OpenSCAD: Make clearing field undoable

Also renames the clear button for clarity. Fixes #8576.

src/Mod/OpenSCAD/OpenSCADCommands.py

@@ -338,7 +338,7 @@ class AddSCADWidget(QtGui.QWidget):
         self.textMsg.setMaximumHeight(h)
         self.textMsg.resize(self.textMsg.width(),h)
         self.buttonadd = QtGui.QPushButton(translate('OpenSCAD','Add'))
-        self.buttonclear = QtGui.QPushButton(translate('OpenSCAD','Clear'))
+        self.buttonclear = QtGui.QPushButton(translate('OpenSCAD','Clear code'))
         self.buttonload = QtGui.QPushButton(translate('OpenSCAD','Load'))
         self.buttonsave = QtGui.QPushButton(translate('OpenSCAD','Save'))
         self.buttonrefresh = QtGui.QPushButton(translate('OpenSCAD','Refresh'))
@@ -357,7 +357,15 @@ class AddSCADWidget(QtGui.QWidget):
         self.setLayout(layout)
         self.setWindowTitle(translate('OpenSCAD','Add OpenSCAD Element'))
         self.textEdit.setText(u'cube();')
-        self.buttonclear.clicked.connect(self.textEdit.clear)
+
+        def undoable_clear():
+            """Clears the textEdit in a way that allows undo of the action"""
+            self.textEdit.setFocus()
+            self.textEdit.selectAll()
+            keypress = QtGui.QKeyEvent(QtGui.QKeyEvent.KeyPress, QtCore.Qt.Key_Delete, QtCore.Qt.NoModifier)
+            QtGui.QGuiApplication.sendEvent(self.textEdit, keypress)
+
+        self.buttonclear.clicked.connect(undoable_clear)
 
     def retranslateUi(self, widget=None):
         self.buttonadd.setText(translate('OpenSCAD','Add'))
@@ -421,9 +429,9 @@ class AddSCADTask:
 
     def loadelement(self):
         filename, _ = QtGui.QFileDialog.getOpenFileName(
-            parent=self.form, 
-            caption='Open file', 
-            dir='.', 
+            parent=self.form,
+            caption='Open file',
+            dir='.',
             filter='OpenSCAD Files (*.scad)'
         )
 
@@ -435,9 +443,9 @@ class AddSCADTask:
 
     def saveelement(self) :
         filename, _ = QtGui.QFileDialog.getSaveFileName(
-            parent=self.form, 
-            caption='Open file', 
-            dir='.', 
+            parent=self.form,
+            caption='Open file',
+            dir='.',
             filter='OpenSCAD Files (*.scad)'
         )