Show spinner for all files when deleting all

Regression introduced by code changes related to "select all" feature.

To test:
1. Open folder containing many files
2. Click "select all" checkbox
3. Delete: spinner appears for all files
4. Go to trashbin
5. Click "select all" checkbox.
6. Delete: spinner appears for all files

Please review @schiesbn @jancborchardt @MorrisJobke