Fixes #2601 - Adds ability to filter TreeView branches/leaves

[tznind]

Fixes #2601 - Adds ability to filter TreeView branches/leaves (e.g. free text search).

I've created this targeting develop as it has quite a small footprint and its a really nice to have core feature I think.

It should be pretty easy to cherry pick to v2_develop

Because a tree is hierarchical its important to consider whether a filter matches the parent or child too. I went for greatest matching so:

• If a branch matches then all sub branches/leaves show even if they don't match the filter
• If a leaf/branch matches all its parents are automatically shown (to expose the matching child)

As with all things in TableView it is only the expanded nodes that get processed. If a node is not expanded its children are unknown (so not considered for matching). This is because of how the table builds itself as the user expands rather than loading everything up front (which can be impossible for some data structures - e.g. file system).

Draft as now so I can write some tests

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

[tznind]

Ok after a bit of trial and git horror I have squashed. to 9c11f61

I will open another PR to cherry pick into v2_develop

Test failure seems unrelated (virtual key stuff)