Dcoraid

- fix: missed a case for task dataset IDs that don't exist but
have been stored in the persistent task dictionary

- feat: allow to add datasets to a collection (11)
- fix: allow empty lines in PersistentTaskDatasetIDDict files
- fix: properly handle task dataset IDs that don't exist (19)

- enh: improve spacing in GUI design (44)
- enh: make sure there are no uploads without RT-DC files
- fix: download in "My Data" tab did not work (42)
- ref: remove old code that cleared old cache and
get rid of the appdirs dependency
- setup: make pyqt5 an optional dependency (39)

- fix: skip passing emtpy-valued API-Key (41)
- enh: added logging capabilities (18)

- setup: bump dclab from 0.38.1 to 0.38.2 (40)

- ref: migrate to package_revise for updating datasets (28)
- setup: bump dclab from 0.37.2 to 0.38.1