Adding a sample for an editable TreeView

[h0lg]

In this PR I'm trying to raise a few questions and issues I encountered while working with the TreeView and AutoCompleteBox widgets.

It adds a third tab hosting a sample of an editable TreeView to the Gallery app's TreeViewPage that features adding, removing, renaming and filtering items while preserving tree node expansion. While I got that working, the solution involves a XAML style with a Binding that I don't like much.

As a follow-up to #231, it includes a working example of focusing an added TextBox - but trying the same on an AutoCompleteBox doesn't seem to work unless hitting a magic breakpoint during debugging. See commit changing input to AutoCompleteBox, breaking focus.

Note that it includes commits that run the EditableTreeView from the Playground app built only for dotnet for quicker testing - because building the Gallery takes a while. Those changes are reverted in the last commit.

You should be able to check out different commits to reproduce the different stages of broken and working features as described in the commit messages.

Please find my open questions in the commit added open questions.

Thank you for your time and consideration 🙏

[h0lg]

@edgarfgp Thanks for the new model to try out and taking the time to answer! I've rebased on your changes, added your suggestion, applied some fixes and tested. Sadly, it's not quite working as it should.

I suspect two major issues:

1. nested TreeViewItems with the IsExpanded being bound to the inner one when it should be bound to the outer
2. the binding via isExpanded() seems to support one-way only

You'll find my detailed feedback and some code helping to showcase the nesting in the latest commit.

[edgarfgp]

I suspect two major issues:

1. nested TreeViewItem with the IsExpanded being bound to the inner one when it should be bound to the outer
2. the binding via isExpanded() seems to support one-way only

Yeah I thought Avalonia itself will manage the TreeViewItem.IsExpanded but seems like we are forced to use .styles([ "avares://Gallery/Styles/EditableTreeView.xaml" ]) for now.

IsExpanded uses already BindingMode.TwoWay, but my guess is that is designed and implemented in a way that is xaml specific.

[edgarfgp]

Thanks for creating this sample. I would be ok merging this as the sample has improved and also the attachDevTools are handy.

[h0lg]

@edgarfgp Thanks for fixing the parameter name/doco mismatch and replacing Option with ValueOption - the latter was new to me.

[h0lg]

@edgarfgp Thank you for your time and effort! I'll clean the unnecessary code from the new sample in a separate PR.