Add combo box and list box controls

[glopesdev]

This PR adds support for the ComboBox and ListBox controls, allowing to select items from a collection using either a list or a drop-down. Each of the controls comes in two flavors:

• Static collection: in this case, the items are a collection of strings which can be edited in the properties, useful for rapid prototyping and simple applications;
• Dynamic data source: in this case, the items in either the combo box or list box are specified from a dynamic sequence of collections of arbitrary objects, with an optional DisplayMember.

In both flavors, notifications are sent whenever the currently selected item changes. Dynamic data sources can be used to allow selection between complex domain objects, e.g. dynamically created observable sequences with a labeled key.