Add Multi Selection to `ComboBox`

`ListBox` has `SelectionMode` property, which can be set to `Multiple` or `Extended`. `ComboBox` shared the `CanSelectMultiple` property, which is set to `false` initially. Setting it to `true` would require:

* the popup to stay open,
* each ComboBoxItem to be rendered as CheckBox to be checked and unchecked,
* to render also a three state CheckBox in the closed DropDown in order to select / unselect all.

It might make sense to make ComboBox inherit `MultiSelector`.

I could also implement the feature, if I get some guidance (e.g. on which branch to implement).