Dtool-lookup-gui

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- Introduced tree view of README.yml content
- Added `dtool-gui` script call
- Modified URL in about dialogue

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- Login window
- Pagination for dserver lookup results
- Sorting for dserver lookup results
- Add/delete tags in GUI
- Add/delete annotations in GUI (key - value pairs)

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- Item download settings (open after download, default download directory)
- New icons
- Fixed issue concerning duplicate local base URI entries

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- Offer DMG image for macOS
- Bundle xpm icons with all distributions

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- Handle errors in dataset copy child processes correctly
- Include dtool icons and according licenses
- Release macOS .app and Windows installer
- Desktop launcher
- Preserve non-ASCII characters in README.yml content received from lookup server
- Bump to ``dtool-s3==0.14.1``, replaces ``DTOOL_S3_DATASET_PREFIX`` by bucket-specific ``DTOOL_S3_DATASET_PREFIX_<BUCKET NAME>``

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- Button 'Show' opens file system browser in frozen executables as well, even on Windows