Create ComboBox component

[rock3r]

A combobox is a dropdown control that also allows you to type in a value. An example is the font size picker in the IDE Appearance settings:

As far as I can tell, Swing doesn't pre-select matching elements in the dropdown and doesn't offer auto-complete — except sometimes it does pre-select matching elements:

(on the same page)

We should offer the ability to pre-select one element. Only single-item selections are allowed. We should also allow the user to set a validation outline.