CodeAssist: Use settable property for IAssistProposal::isFragile

Removes the need to create yet another subclass (for overriding a
virtual method) in some situations.

Change-Id: I55d70de60b4f8d127a175d996f797700c2f172a6
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/cppeditor/cppvirtualfunctionassistprovider.cpp

@@ -106,9 +106,9 @@ public:
                             bool openInSplit)
         : GenericProposal(cursorPos, items)
         , m_openInSplit(openInSplit)
-    {}
-
-    bool isFragile() const override { return true; }
+    {
+        setFragile(true);
+    }
 
     IAssistProposalWidget *createWidget() const override
     { return new VirtualFunctionProposalWidget(m_openInSplit); }