More protection around an exceedingly dangerous hack.

The API is needed for wxFormBuilder, and I couldn't figure out any
way around it, but we can at least reduce the risk of someone calling
it accidentally (again).

(cherry picked from commit 46564055f4)

bitmap2component/bitmap2cmp_gui.cpp

@@ -147,6 +147,7 @@ END_EVENT_TABLE()
 BM2CMP_FRAME::BM2CMP_FRAME( KIWAY* aKiway, wxWindow* aParent ) :
     BM2CMP_FRAME_BASE( aParent )
 {
+    m_ident = FRAME_BM2CMP;      // Initialized to wxID_ANY by wxFormBuilder
     SetKiway( this, aKiway );
 
     for( wxString unit : { _( "mm" ), _( "Inch" ), _( "DPI" ) } )