Replace TreeView with TreeDataGrid

[Schanze97]

Replaced the TreeView with the TreeDatagrid for the ProjectExplorer and LibraryExplorer

This involves the following changes

• added custom theme
• SelectedElements are only bindable as IReadOnlyList. Therefore a new function for selecting nodes has been added. This function computes the IndexPath of the item, and selects it from that
• IsExpandable is not bindable. If it is binded in the style, a measuring exception will occur. It is bound manually the following way:
  • to source -> bound ToggleButton in ControlTemplate to set the property in the ViewModel
  • to view -> added attached Property to sync the view with the Vm

Fixes:

• when searching for an element the default behavior was, that the TreeView collapsed and expanded to the found elements. collapsing and expanding causes the lazy loading (LoadingDummyNode -> ProjectExplorerNode) to execute. After that, the found elements in the search result is no longer the valid object reference in the view. The function has been changed, that only loaded nodes are taken to account for the search.