Summary:
- New UI for the modal look mostly following the design prepared previously.
- Use <object> for rendering PDFs (on a Mac works on Firefox, Chrome, Safari; needs checking on Windows)
- While we are at it, use <video> and <audio> for relevant files (object would
render them too, but without the option to disable autoplay).
- Use <object> for unknown types, except for text/html (unsafe) and other text
types (need more work to render well).
- Fix skipping save on Escape or when attachments are unsaved (previously a
noop action was emitted, creating surprises with undo).
- Display extension for files without preview, both in-cell and in the modal.
- Replace tiny "eye" icon to preview particular attachment with double-clicking.
- As an accidental feature, a particular attachment can be previewed by typing 1, 2, 3, etc into cell.
- Renamed PreviewsWidget/PreviewModel to AttachmentsWidget/AttachmentsEditor.
Test Plan: Unified old and new tests for attachments, added new test cases.
Reviewers: paulfitz
Reviewed By: paulfitz
Differential Revision: https://phab.getgrist.com/D2667
