1. changed wxControl::GetLabel() to return the originally provided label and

1. changed wxControl::GetLabel() to return the originally provided label and
   not the one stripped from mnemonics (this was inconsistent with the other
   ports and resulted in problems when using wxUpdateUIEvent::SetText())
2. added wxControl::GTKConvertMnemonics(), GTKRemoveMnemonics() and also
   helper GTKSetLabelForLabel() and GTKSetLabelForFrame() wrappers
3. use them instead of duplicating their code in different derived controls

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@36435 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775