Add wxWithImages::GetImageCount() and HasImages() helpers

These functions allow avoiding using wxImageList explicitly in a couple
of places.

No real changes, this just prepares for adding new API not based on
wxImageList.

samples/notebook/notebook.cpp

@@ -157,9 +157,9 @@ wxPanel *CreateInsertPage(wxBookCtrlBase *parent)
 
 int GetIconIndex(wxBookCtrlBase* bookCtrl)
 {
-    if (bookCtrl && bookCtrl->GetImageList())
+    if (bookCtrl)
     {
-       int nImages = bookCtrl->GetImageList()->GetImageCount();
+       const int nImages = bookCtrl->GetImageCount();
        if (nImages > 0)
        {
            return bookCtrl->GetPageCount() % nImages;