Clang: Remember selected function signature hint

...when typing more arguments:

    struct Foo {};
    void f(int, int);
    void f(Foo, Foo);
    void f(char, char);

    void c()
    {
        f( // 1. Trigger completion with Ctrl+Space
           // 2. Chose item "f(Foo, Foo)"
           // 3. Type: Foo(),
           // OK, signature hint "f(Foo, Foo)" is displayed again
    }

FunctionHintProposalWidget and IFunctionHintProposalModel are
instantiated for each calculation, so remember the selected hint in the
CodeAssist. Keep the latest 20 entries.

Task-number: QTCREATORBUG-11688
Change-Id: I579fc6d8a35dd8fa398e4b3170ddc05a85252d1a
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/texteditor/codeassist/functionhintproposalwidget.h

@@ -63,6 +63,9 @@ private:
     void updatePosition();
     void abort();
 
+    int loadSelectedHint() const;
+    void storeSelectedHint();
+
 private:
     FunctionHintProposalWidgetPrivate *d;
 };