Searchable CheckComboBox

Is your feature request related to a problem? Please describe.
I looked into HandyControl to solve a problem I'm facing where I'll need to have a combobox, populated with hundreds of items, which allows me to select multiple items. This is achieved using the CheckComboBox from Handy Control, however, because the element is populated with hundreds of items, I'll need to be able to search trough all the items instead of the user scrolling up and down looking for the desired item.
I've tried to use the property IsTextSearchEnabled set to true but that still does not allow me to search the items.

Describe the solution you'd like
Implement a SearchEnabled or use the already present IsTextSearchEnabled property which when set to true will provide a search box, which would be the first item of the list ( same position as the Select All ) allowing to search trough the items of the CheckComboBox. this search should be performed using a string.Contains instead of string.StartsWith .

Describe alternatives you've considered
If providing a search box is not possible use the same mechanism as the default combobox element, where the user would type directly in the combobox the value to be search and the list would show all items that are returned with that search. The search should still be done using string.Contains instead of string.StartsWith

Thanks for the amazing work done with this package!

Start by locating the CheckComboBox implementation and the existing IsTextSearchEnabled behavior in the HandyControl source. Review related control tests or examples if present. Done means the control can search hundreds of items using substring matching while retaining multi-selection and the requested search-box or typing interaction.