Implement MultiSelectTreeView control

[zspitz]

Currently using https://unclassified.software/en/source/multiselecttreeview, and I also looked at https://github.com/cmyksvoll/MultiSelectTreeView

Required features:

• [ ] Multiple selection support
• [ ] .NET Core support
• [ ] Enable applying focused styling even when the control does not have the focus (like we do for built-in WPF controls)
• [ ] NuGet package (alternatively, include the control in ZSpitz.Utils.Wpf)

Nice to have:

• [ ] Selected Values and SelectedValuePath methods
• [ ] Allows simply assigning items/values to SelectedItems / SelectedValues (see below)
• [ ] No editing not required or fancy selection behavior; makes it easier to debug incompatibility issues
• [ ] Optional lazy loading support (although that could be part of the view-model if need be)

[zspitz]

Multiple DLLs are no longer an issue, as we are using Costura to load dependent DLLs. We are currently using MultiSelectTreeview. There are still two remaining issues:

• I haven't figured out how to style the selection to match other controls #81
• The control targets .NET Framework only, and we want something that multitargets .NET Core and .NET Framework.

[zspitz]

I've requested fixes for these issues:

• https://github.com/ygoe/MultiSelectTreeView/issues/51
• https://github.com/ygoe/MultiSelectTreeView/issues/52

and here is another alternative multiple-selection treeview:

• https://github.com/dotnetprojects/TreeViewEx/issues/3

[zspitz]

Another possible multi-selection treeview implementation: https://github.com/PropertyTools/PropertyTools