QmlDesigner.propertyEditor: hide reset property button if not applicable

See Bauhaus-233

share/qtcreator/qmldesigner/propertyeditor/Qt/ExtendedFunctionButton.qml

@@ -62,8 +62,9 @@ QToolButton {
     actions:  [
     QAction {
         text: qsTr("Reset")
+        visible: backendValue.isInSubState || backendValue.isInModel
         onTriggered: {
-            backendValue.resetValue();
+            backendValue.resetValue();            
         }
 
     },

share/qtcreator/qmldesigner/propertyeditor/Qt/propertyEditor.css

@@ -375,7 +375,7 @@ QMenu {
      background-color: #494949;
      border: 1px solid black;
      margin: 0px;
-     color: #cacaca;
+     color: #dadada;
  }
 
  QMenu::item {
@@ -390,6 +390,10 @@ QMenu {
      border-radius: 4px;
      padding: 2px 25px 2px 20px;
  }
+ 
+  QMenu::item:disabled {    
+    color: #aaaaaa;
+ }
 QMenu::separator {
      height: 1px;
      background: #404040;