Add wxTreeCtrl::GetStateImageCount() and HasStateImages()

Add these functions for consistency with SetStateImages() and to avoid
using GetStateImageList() in the treectrl sample, which can now use
GetStateImageCount() instead.

samples/treectrl/treetest.cpp

@@ -1288,7 +1288,7 @@ void MyTreeCtrl::DoToggleState(const wxTreeItemId& item)
 
         srand (time(nullptr));
         do {
-            nState = rand() % GetStateImageList()->GetImageCount();
+            nState = rand() % GetStateImageCount();
         } while (nState == state);
 
         SetItemState(item, nState);