Get rid of QtCreateControl() from wxQt code base

If possible, all windows should call wxWindow::Create() on creation now,
because among other things:
- It manages generic and non-generic windows uniformly and transparently
- It ensures an appropriate initial size for the window
- AddChild() and PostCreation() are automatically called
- Scrollbar policies are set in one place.

include/wx/qt/window.h

@@ -66,7 +66,8 @@ public:
                 long style = 0,
                 const wxString& name = wxASCII_STR(wxPanelNameStr));
 
-    // Used by all window classes in the widget creation process.
+    // Derived classes have to call PostCreation() explicitly if they don't call
+    // our Create() method during widget creation process.
     void PostCreation( bool generic = true );
 
     void AddChild( wxWindowBase *child ) override;
@@ -202,6 +203,8 @@ protected:
     virtual void DoSetClientSize(int width, int height) override;
     virtual void DoGetClientSize(int *width, int *height) const override;
 
+    virtual wxSize DoGetBestSize() const override;
+
     virtual void DoMoveWindow(int x, int y, int width, int height) override;
 
 #if wxUSE_TOOLTIPS