Mobile: Support importing from JEX files

[personalizedrefrigerator]

Summary

Related forum post: https://discourse.joplinapp.org/t/import-jex-file-on-android/31126/8

This pull request adds support for importing from .JEX files on mobile.

Notes

This pull request is mostly UI changes (InteropService.instance().import already works for .jex files on mobile). Specifically, it:

1. Renames the "Export" section in settings to "Import and Export"
2. Creates a new TaskButton.tsx file that handles displaying progress and warnings for both export and import tasks.
3. Adds a new import button that uses TaskButton.

Note that it now also possible to do multiple exports without closing and re-opening the export settings tab.

Currently, imported notebooks are added to the top level (not added within a notebook). It may make sense to change this, for example, by creating a new notebook for each import.

Screenshots

Note: The foo warning was added as a demonstration of the warning UI:

Testing

1. Ensure that at least one image and emoji or other non-ASCII character is present in a note.
2. Export all notes as JEX from the "Import and Export" settings screen.
3. Import from the just-exported JEX file.
4. Verify that notebooks and notes have been duplicated.
5. Verify that the image and emoji from step 1 are present in the duplicated (imported) note.

This has been tested successfully on an Android 12 emulator.

[personalizedrefrigerator]

Currently, we have a similar Platform.select in most places we use pickDocument. It may make sense to move this into the implementation of pickDocument.

[laurent22]

That's a good idea, would you mind doing this refactoring? Maybe with some wrapper over pickDocument

[personalizedrefrigerator]

Note.tsx currently relies on pickDocument producing the same output as the image picker library:

joplin/packages/app-mobile/components/screens/Note.tsx

Line 764 in ce67291

const localFilePath = Platform.select({

Doing this refactor would likely cause this assumption to break. Refactoring pickDocument should be done while also refactoring Note.tsx.