Support switching pakfile explorer between tree and flat view

[tsa96]

Add a button to the top bar of the pakfile explorer that toggles between showing the pakfile contents as a tree (the current system), or a flat list.This used to be possible with the old pakfile UI and it was really nice as a way to quickly glance over the contents of a pakfile.

In theory it's quite easy to do, just create a FlatTreeDataGridSource as well as the current HierarchicalTreeDataGridSource then expose a reactive property that points either of the two (probably best to lazily construct the flat tree for perf).

There's a PR into the TreeDataGrid repo discussing "making this easier", which was never merged, I think because some of the changes made it in as part of a separate PR - worth taking a look at. Also be sure to see https://github.com/AvaloniaUI/Avalonia.Controls.TreeDataGrid/tree/master/samples/TreeDataGridDemo for example usage.

This is mostly blocked until #39 is merged. Someone could get started if they want, but pakfile explorer code on that branch may change.