Nice contribution,

Things to try:
-easy ability to change the optionbutton and checkbox image,
-until an item is selected on a listbox or combobox, it should return -1 for the selection instead of 0,
-add a function for returning the selected line number as well as the string.
-change your variable types from var to short (some people will say there's not much impact, but there's a lot of vars and you never know how many of your tools the end-user will create)
-add use of scrollbars to listboxes and comboboxes
-add a sort function to listboxes and comboboxes