Adds the backspace as a schortcut to trigger the delete object action.

This StandardKey only works for MacOS, for more info see http://doc.qt.io/QtVirtualKeyboard/qml-qtquick-enterprise-virtualkeyboard-backspacekey.html

fixes #237

resources/qml/Actions.qml

@@ -119,6 +119,7 @@ Item {
         //: Delete object action
         text: qsTr("Delete Object");
         iconName: "edit-delete";
+        shortcut: StandardKey.Backspace;
     }
 
     Action {
@@ -126,7 +127,7 @@ Item {
         //: Center object action
         text: qsTr("Center Object on Platform");
     }
-    
+
     Action
     {
         id: groupObjectsAction