Return correct string from wxEVT_TEXT wxComboBox events.

wxCommandEvent::GetString() could return empty string for the
programmatically-generated wxEVT_TEXT events from a wxComboBox.

Fix this by extending the on-demand string retrieval in wxCommandEvent to
wxComboBox as well (it was done only for wxTextCtrl).

Closes #3901.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@77056 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775