CP-6083: Preliminary tidy up: moved all custom comboboxes into a subfolder of the Controls folder.

Signed-off-by: Konstantina Chremmou <konstantina.chremmou@citrix.com>
CP-6083: Preliminary refactoring: moved common code responsible for skipping non
selectable combobox items from the classes EnableableComboBox and ISODropDownBox
to a base class called NonSelectableComboBox.

Signed-off-by: Konstantina Chremmou <konstantina.chremmou@citrix.com>
CP-6083: Updated XenModel with latest C# bindings.

Signed-off-by: Konstantina Chremmou <konstantina.chremmou@citrix.com>