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 Use the same selection style across treeview, source code and datagrids #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:

• .NET Core support ygoe/MultiSelectTreeView#51
• Keep selected style even when not focused ygoe/MultiSelectTreeView#52

and here is another alternative multiple-selection treeview:

• Package for NuGet dotnetprojects/TreeViewEx#3

[zspitz]

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