Dcoraid

- fix: KeyError when checking for existence of resource (24)
- enh: drag&drop files to manual upload dialog (23)
- enh: set upload rate refresh interval to >3s (22)
- enh: permit to only update resource schema supplements when
executing `dataset.add_resource` with `exist_ok=True`

- fix: removed confusing wait-cursor during circle selection
- fix: manual upload dialog was shown even though user pressed
cancel on circle creation (20)
- fix: silenced very long error message about connection errors
in the background thread that refreshes the login status
- tests: add complementary tests for upload queue and persistent job
list

- fix: TypeError when attempting a manual upload when the user did not
have a circle set-up (17)
- fix: manual uploads were not added to persistent job list

- fix: introduce LocalTaskResourcesNotFoundError in upload.task
which results in a custom DCORAidQueueWarning in upload.queue
in case a dataset resource is not available at the moment;
previously, this scenario resulted in a FileNotFoundError
- fix: when loading tasks recursively from a directory, only ask
once for the circle (if there are multiple available to the user)
- fix: immortalized jobs that fail to be summoned (e.g. because the
user moved the resources to a different directory), can now be
imported again from the orgiginal (albeit moved) .dcoraid-task file

- tests: fix test logic for new anonymous test (15)

- fix: UnboundLocalError "progress" referenced before assignment,
because "wait-disk" status was not handled
- fix: UploadQueue could not handle jobs in PersistentUploadJobList that
had been deleted on the server
- fix: Initial startup and anonymous mode did not work, because
there was an attempt to access the API (15)