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.

interface/wx/withimages.h

@@ -18,6 +18,27 @@ public:
     wxWithImages();
     virtual ~wxWithImages();
 
+    /**
+        Return the number of images in this control.
+
+        The returned value may be 0 if there are no images associated with the
+        control.
+
+        @see HasImages()
+
+        @since 3.1.6
+     */
+    int GetImageCount() const;
+
+    /**
+        Return true if the control has any images associated with it.
+
+        @see GetImageCount()
+
+        @since 3.1.6
+     */
+    bool HasImages() const;
+
     /**
         Sets the image list for the page control and takes ownership of the list.