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/iassistproposalwidget.cpp

@@ -55,6 +55,16 @@ IAssistProposalWidget::IAssistProposalWidget()
 IAssistProposalWidget::~IAssistProposalWidget()
 {}
 
+int IAssistProposalWidget::basePosition() const
+{
+    return m_basePosition;
+}
+
+void IAssistProposalWidget::setBasePosition(int basePosition)
+{
+    m_basePosition = basePosition;
+}
+
 /*!
     \fn void TextEditor::IAssistProposalWidget::setAssistant(CodeAssistant *assistant)