Implement combobox basic methods (adding internal wxQtComboBox helper), very similar to wxChoice implementation to wxQT. Note that the old wxQtComboBox superseded and no more shared with wxChoice
Remove combobox_qt header as it is no longer used (helper implementations are now internal to wxChoice and wxComboBox)
Implement combobox event (fixing wxQtComboBox helper signal handler), it should use SendSelectionChangedEvent but wxControlWithItemsBase collides with wxComboBoxBase, and it cannot derive from wxChoice as in wxGTK as in wxQT wxTextEntry is not completely implemented. So, the current implementation is not optimal (it has some minor code duplication) but it works ok