On wxGTK2 the label woudl not be realigned if SetLabel was called with

a new label.  Changed it to use gtk_button_set_label so that the
widget would care of doing it, and also to create with widget with
gtk_button_new_with_mnemonic so the mnemonic behaviour would be
preserved.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@27589 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775