[Feature] Folder Mode

- [x] Persist open folder
- [x] Allow switching folder
- [ ] Allow closing folder
- [ ] Test more...
- [ ] DataSource loaded from file context in a XAML page doesn't show up in the Data Sources Activity... (until preview refreshed)
- [ ] Editing/saving data source in panel doesn't save back to the data source file...
- [ ] Document capabilities... (Image file loading, data context loading, and resources [kind of])
- [x] Investigate loading resource file from a merged dictionary/resources thing... (should work like other injections, Maybe for first pass it doesn't load edits if resource open in other tab, that'd be another pass later?)