Renamed Chromium Explorer -> Chromium Source Explorer, and extracted generic ToolWindow code to a parent folder.
Move SourceExplorerRootNodes down to the common level.
Extract some common logic into ViewModelBase.
Add Chromium Build Explorer tool window.

This feature will continue to be inaccessible (by way of an invisible
menu item) while under development.
Delete TreeViewItemViewModelHost.
Merge commit 'a857150e1a75e90c59a2283c1ac9ce552035e427' into build_explorer
Add installation discovery to build explorer.
Add Installation discovery to Build Explorer.