Support adding/updating/removing attachments

[Frederick888]

I'd still like to leave as much as heavy-lifting in the native messaging host. It should provide the extension with what it needs to call compose APIs.

[Frederick888]

e.g. X-ExtEditorR-Attach: /absolute/path/to/file.zip

If user wants to use a different name, we can support something like PKGBUILD: X-ExtEditorR-Attach: another-name.zip::/absolute/path/to/file.zip. If the name or path already contains double colons, simply add another colon to the separator: X-ExtEditorR-Attach: another::name.zip:::/absolute/path/to/file::name.zip (or four colons as separator if triple's already in name/path, and so forth.)

[Frederick888]

It seems that I have to fully load attachments into memory to achieve this cos addAttachment() only accepts File objects, and it's not possible to create Files from file paths using pure JavaScript. So I can at best support renaming/removing attachments only, which is next to being useless.

Unless there is some workaround this is parked indefinitely.