Implement GetStringSelection() in generic wxBitmapComboBox

Also update the test to avoid assuming that wxBitmapComboBox inherits
from wxComboBox, which now allows it to build (and pass) on all
platforms.

Closes https://github.com/wxWidgets/wxWidgets/pull/2057

src/generic/bmpcboxg.cpp

@@ -135,6 +135,11 @@ wxBitmapComboBox::~wxBitmapComboBox()
     DoClear();
 }
 
+wxString wxBitmapComboBox::GetStringSelection() const
+{
+    return wxItemContainer::GetStringSelection();
+}
+
 // ----------------------------------------------------------------------------
 // Item manipulation
 // ----------------------------------------------------------------------------