Add keyboard shortcuts for grouping/ungrouping

Contributes (in a way) to CURA-1054, \sa CURA-1531

resources/qml/Actions.qml

@@ -176,6 +176,7 @@ Item
         text: catalog.i18nc("@action:inmenu menubar:edit","&Group Objects");
         enabled: UM.Scene.numObjectsSelected > 1 ? true: false
         iconName: "object-group"
+        shortcut: "Ctrl+G";
     }
 
     Action
@@ -184,14 +185,16 @@ Item
         text: catalog.i18nc("@action:inmenu menubar:edit","Ungroup Objects");
         enabled: UM.Scene.isGroupSelected
         iconName: "object-ungroup"
+        shortcut: "Ctrl+Shift+G";
     }
-    
+
     Action
     {
         id: mergeObjectsAction
         text: catalog.i18nc("@action:inmenu menubar:edit","&Merge Objects");
         enabled: UM.Scene.numObjectsSelected > 1 ? true: false
         iconName: "merge";
+        shortcut: "Ctrl+Alt+G";
     }
 
     Action

resources/qml/Cura.qml

@@ -680,6 +680,7 @@ UM.MainWindow
         MenuItem { action: actions.reloadAll; }
         MenuItem { action: actions.resetAllTranslation; }
         MenuItem { action: actions.resetAll; }
+        MenuSeparator { }
         MenuItem { action: actions.groupObjects; }
         MenuItem { action: actions.mergeObjects; }
         MenuItem { action: actions.unGroupObjects; }
@@ -692,6 +693,7 @@ UM.MainWindow
         MenuItem { action: actions.reloadAll; }
         MenuItem { action: actions.resetAllTranslation; }
         MenuItem { action: actions.resetAll; }
+        MenuSeparator { }
         MenuItem { action: actions.groupObjects; }
         MenuItem { action: actions.mergeObjects; }
         MenuItem { action: actions.unGroupObjects; }