* lib/fs, lib/model: Improve filesystem operations during tests (fixes#5422)
Introduces MustFilesystem that panics on errors and should be used for operations
during testing which must never fail.
Create temporary directories outside of testdata.
* don't do a filesystem, just a wrapper around os for testing
* fix copyright
Adds a receive only folder type that does not send changes, and where the user can optionally revert local changes. Also changes some of the icons to make the three folder types distinguishable.
