[WPF] Entry ComboBox backend

[ermau]

WPF implementation of IComboBoxEntryBackend

[ermau]

The only remaining thing is that a custom set of views breaks item selection such that you will get Object[] in the text box instead of the column set by SetTextColumn. The only real way to set what goes into that text box is to set DisplayMemberPath, which conflicts with the item template created for custom views.