Combine combobox filter with items/CollectionView filter

[kvpt]

This library work very well with simple list. But in my case I also want to use it with a filtered CollectionView. In this case, on the first opening the combobox is correctly filtered, but after the first search, the combobox overwrite the filter defined in the CollectionView. I think the right thing to do is to combine the two filters. If this change is too breaking for you, we can introduce a new setting in the dedicated class.

[vain0x]

Motivation is reasonable and the changes seem not breaking. Thank you!