Use backspace as default to delete on mac

Only full size mac keyboards has a specific delete key, most doesn't.

src/Gui/CommandDoc.cpp

@@ -1318,7 +1318,11 @@ StdCmdDelete::StdCmdDelete()
   sWhatsThis    = "Std_Delete";
   sStatusTip    = QT_TR_NOOP("Deletes the selected objects");
   sPixmap       = "edit-delete";
+#ifdef FC_OS_MACOSX
+  sAccel        = "Backspace";
+#else
   sAccel        = keySequenceToAccel(QKeySequence::Delete);
+#endif
   eType         = ForEdit;
 }