DCOR-Aid 0.18.2

Most work has gone into refreshing the user interface. I added a local database cache, which means searching for datasets is now much faster. Local database upgrades are incremental which means that initial database creation may take some time (depending on network and CPU speed). I removed the non-functional "Sharing" tab. In the future I plan to incorporate sharing-related functionalities in the "My Data" tab. A few minor glitches have also been fixed.

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• feat: implement local database for caching (partially #5)
• fix: missing .rtdc resource files in filter view
• fix: error handling when connection to server fails on download (#75)
• fix: '_condensed' suffix for file stem randomly missing in downloads
• fix: finalize/ephermalize job when upload finishes when widget not visible
• fix: race condition in DownloadJob.get_status
• enh: display DCOR instance title in search bar
• enh: increase up/download panel refresh rate to 2 Hz
• enh: initialize up/download panel with final column widths
• enh: add tool tip with full resource and dataset name in up/download panel
• enh: make icons "grayer" so they work in dark themes as well
• enh: add SQLiteKeyJSONDatabase for persistent metadata storage
• enh: add MetaCache for persistent metadata management
• enh: add CKANAPI.hostname
• enh: sort search results by creation date instead of score and modified date
• enh: convenience kwarg since_time for search_dataset_via_api
• enh: implement DBExtract.__repr__
• enh: remove unused "Sharing" page
• enh: add icon for "Find Data"
• enh: cache CKANAPI instance in GUI
• enh: APIInterrogator.search_dataset_via_api optionally returns list
• enh: DBExtract.__iadd__ supports lists
• ref: remove mode from DBInterrogator
• ref: replace user_list with user_autocomplete
• ref: replace search_dataset with search_dataset_via_api
  in APIInterrogator; search_dataset is now only free text search
• ref: get_collections and get_circles now return list of dictionaries
• ref: rename gui submodules for better overview
• ref: move "My Data" panel into its own submodule
• ref: implement is_dc_resource_dict
• tests: allow to start test session without internet connection