Fix accessibility of some group commands

Several group commands are active but require an active document to work. This PR overrides the method isActive() to
disable the commands if no active document exists.

The affected commands are:
* Std_ViewGroup
* Std_LinkActions
* PartDesign_CompDatums
* PartDesign_CompSketches

src/Gui/CommandLink.cpp

@@ -892,7 +892,15 @@ public:
         addCommand(new StdCmdLinkImportAll());
     }
 
-    const char* className() const override {return "StdCmdLinkActions";}
+    const char* className() const override
+    {
+        return "StdCmdLinkActions";
+    }
+
+    bool isActive() override
+    {
+        return hasActiveDocument();
+    }
 };
 
 //===========================================================================

src/Gui/CommandView.cpp

@@ -1648,6 +1648,11 @@ public:
     {
         return "StdCmdViewGroup";
     }
+
+    bool isActive() override
+    {
+        return hasActiveDocument();
+    }
 };
 
 //===========================================================================

src/Mod/PartDesign/Gui/Command.cpp

@@ -2422,6 +2422,11 @@ public:
     {
         return "CmdPartDesignCompDatums";
     }
+
+    bool isActive() override
+    {
+        return hasActiveDocument();
+    }
 };
 
 // Command group for datums =============================================
@@ -2452,6 +2457,11 @@ public:
     {
         return "CmdPartDesignCompSketches";
     }
+
+    bool isActive() override
+    {
+        return hasActiveDocument();
+    }
 };
 
 //===========================================================================