DUI: Fix StandardPanel

Priority: _Critical_
Ok, something fix, that something will be broken... that's my style!
I fixed class button [here](https://github.com/Benglin/Dynamo/pull/186), but I didn't think, that if we press member button, we shouldn't handle.

Reviewers
@Benglin ,@vmoyseenko, please take a look.
Please document why is this the case.