Finalize migration flow:
- Add entry to "Add" menu
- Suggest to migrate old profiles when there are no profiles
- Add informational message
- Keep included profiles on top
- Allow deletion of migratable profiles
- Fix duplicated Form in preview
- Rename views and models
Improve some Theme modifiers:
- Empty message with full screen option
- Progress modifier with custom view
- Confirmation dialog with custom message
For some reason, Table doesn't seem to inherit the environment in some
cases. Reapply environment to each TableColumn (only Theme is required).
Work around what clearly seems to be a SwiftUI bug.
Fixes#872
