Fix changing tooltip via wxToolTip::SetTip() in wxQt

Update the actual tool tip used by Qt and not just the internal variable
when SetTip() is called.

Also implement wxToolTip::SetWindow().

Closes https://github.com/wxWidgets/wxWidgets/pull/1175

include/wx/qt/tooltip.h

@@ -10,7 +10,8 @@
 
 #include "wx/object.h"
 
-class wxWindow;
+class WXDLLIMPEXP_FWD_CORE wxWindow;
+
 class WXDLLIMPEXP_CORE wxToolTip : public wxObject
 {
 public:

include/wx/qt/window.h

@@ -173,6 +173,11 @@ public:
 
     virtual QScrollArea *QtGetScrollBarsContainer() const;
 
+#if wxUSE_TOOLTIPS
+    // applies tooltip to the widget.
+    virtual void QtApplyToolTip(const wxString& text);
+#endif // wxUSE_TOOLTIPS
+
 protected:
     virtual void DoGetTextExtent(const wxString& string,
                                  int *x, int *y,

src/qt/tooltip.cpp

@@ -43,12 +43,16 @@
 
 wxToolTip::wxToolTip(const wxString &tip)
 {
+    m_window = NULL;
     SetTip(tip);
 }
 
 void wxToolTip::SetTip(const wxString& tip)
 {
     m_text = tip;
+
+    if ( m_window )
+        m_window->QtApplyToolTip(m_text);
 }
 
 const wxString &wxToolTip::GetTip() const
@@ -59,6 +63,7 @@ const wxString &wxToolTip::GetTip() const
 
 void wxToolTip::SetWindow(wxWindow *win)
 {
+    wxCHECK_RET(win != NULL, "window should not be NULL");
     m_window = win;
-    wxMISSING_FUNCTION();
+    m_window->QtApplyToolTip(m_text);
 }

src/qt/window.cpp

@@ -951,14 +951,22 @@ void wxWindowQt::DoMoveWindow(int x, int y, int width, int height)
 
 
 #if wxUSE_TOOLTIPS
+void wxWindowQt::QtApplyToolTip(const wxString& text)
+{
+    GetHandle()->setToolTip(wxQtConvertString(text));
+}
+
 void wxWindowQt::DoSetToolTip( wxToolTip *tip )
 {
+    if ( m_tooltip == tip )
+        return;
+
     wxWindowBase::DoSetToolTip( tip );
 
-    if ( tip != NULL )
+    if ( m_tooltip )
-        GetHandle()->setToolTip( wxQtConvertString( tip->GetTip() ));
+        m_tooltip->SetWindow(this);
     else
-        GetHandle()->setToolTip( QString() );
+        QtApplyToolTip(wxString());
 }
 #endif // wxUSE_TOOLTIPS