Style TreeViewItem

[grokys]

This PR adds a new style for TreeViewItem to match VS' theme.

• Correctly uses gray as the non-active selection background (see Changes list: wrong colors when highlighting selection #1294)
• Uses the same expander size as elsewhere
• Styles the expanders to be hollow when not expanded and use correct mouseover colors

Before:

After:

Fixes #1294

[jcansdale]

This is looking great. 😄

Being super pedantic, the vertical alignment of the icons isn't quite the same as Solution Explorer.

• Solution Explorer
  

• Changes
  

[grokys]

I think pedanticism is warranted here @jcansdale - that's pretty much what this PR is about ;) Will fix.

[grokys]

@jcansdale ok, adjusted that. It's still not identical to solution explorer (I can't work out how to get both the folder icons and the file icons to align identically - seems to be one or the other!) but it's pretty close.

[meaghanlewis]

This LGTM ✔️

[jcansdale]

Shiny. ❇️