Override Combobox's filter method

[jtruffot]

Currently, the combobox performs case-insensitive filtering. Would it be possible to override this behavior to allow accent-insensitive searches or something more complex?

I managed to implement a solution but as it stands, the ClrOptionItems directive seems impossible to extend properly.

[kevinbuhmann]

Thanks for this report! I made the combobox search diacritic-insensitive in #1210. This change will be released in 15.x, 16.x, and 17.x.