Add ComboBox index lookup methods

Add lookup methods to ComboBox so apps can find the index of an item by display text or by the custom tag stored with SetItemTag. This removes the need for users to manually loop through ItemCount, ItemAt, and ItemTag when using a combo box as a lookup control for database records or similar data.

Proposed API:

IndexOf(text As String) As Integer
IndexOf(text As String, caseSensitive As Boolean) As Integer
IndexOfItemTag(tag As Object) As Integer

All methods should return the first matching index, or -1 when no item matches.