Dtool-lookup-gui

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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

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- About dialog with version info
- Offer multi-line search query editor via drp-down button instead of double click
- Button 'Show' opens file system browser in frozen executables as well
- Display correct number of copied items
- Added missing gir1.2-gtksource-4 system package dependency to Ubuntu build workflow
- Preserve executable bits in release archives
- Don't fail listing proto datasets without README.yml via dtool-s3 (or any other storage broker)

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- Added refresh button
- Handle non-ASCII characters in S3 metadata by using dtool-s3 development version