Add Multi Selection to ComboBox

[AchimStuy]

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).

[miloush]

Related: #8861

Note that you concerns are mostly UI related and that is really the last concern here (and possibly apart from keeping popup open they are questionable as defaults, ListBox also doesn't have check boxes).

Making the controls inherit from MultiSelector is a good start. There will be enough virtualization issues to tackle.