TextEditor: Fix is visible check of function hint

Since the FunctionHintProposalWidget is just a proxy witget that uses a
FakeToolTip it is not shown directly so we cannot check the visibility
with isVisible.

Fixes: QTCREATORBUG-25664
Change-Id: I6888e373afcfc79565ce8e3dad1bb05501d58200
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/texteditor/codeassist/functionhintproposalwidget.h

@@ -52,6 +52,8 @@ public:
     void updateProposal(const QString &prefix) override;
     void closeProposal() override;
 
+    bool proposalIsVisible() const override;
+
 protected:
     bool eventFilter(QObject *o, QEvent *e) override;