Fixes hard-coded widget border size - fixes theming issues

The ComboBox widget never asked the fpgStyle the size of the theme
border, and always assumed 2px, which was wrong. It now correctly
queries the fpgStyle (current theme) for the border sizes.
Fixing Listbox hard-coded border width - fixes theming issues.

As with the ComboBox before, the ListBox hard-coded a 2px border,
instead of querying the fpgStyle for the theme's border widths.