Add "drop down" control

[customlogic]

As in dat.GUI, when the user passes in a list of string/value pairs we create a drop down control.

When not activated, the drop down should show the current state:

When you click the drop down, the list of options should open up. As you point up/down the list, the items highlight as you pass over them. When you "click" the current item is selected. If you point away from the drop down, it closes.

[mflux]

Actually, it still needs to support arrays.

[customlogic]

Can you add a little down arrow to the right of the control (the international sign for "this is a drop down")?

[mflux]

Just for you Jeff