Adding Double Click support to the ComboBox control

I was recently using a `ComboBox` control with the `DropDownStyle` set to `Simple`, effectively turning into a combined text box and list box. However, when I wanted an action to occur on double clicking an item in the list I found that the control doesn't actually offer double click support. I suppose I should have just ripped out the combo box at that point and went with dedicated controls but instead I decided to extend `ComboBox` to support double clicks.

Richard Moss