Project import tree improvements

Following #4805 and #5639 there are some pending items we are considering for the project import tree:

- [x] Imports tree should support duplicate node names (#5711, PR #5736)
- [x] Review whether ordering of children in tree reflects ordering of imports, including in the case of updates that change order (#5754, PR #5736, PR #5775)
- [ ] Browse object showing path, label, group label
- [ ] Option to show these at all times
- [ ] Include imported files in "Find in Files"
- [ ] (Maybe) track "Show all files" state via Dataflow
- [ ] Indicate when, in the import ordering, the project (eg csproj) is evaluated (requires change to MSBuild and may not be feasible)
- [ ] Investigate the 'Search within file contents' option of Solution Explorer's search:
- [ ] Review context menu on the "Imports" node
- [ ] While the import tree is visible, drag/dropping the project's `.csproj.user` file into the editor gives:

> 'ClassLibrary1.csproj.user' cannot be opened. The file is not supported.