Add `ImageProxy#downloadAsIcon`

[freya022]

Pull Request Etiquette

• [X] I have checked the PRs for upcoming features/bug fixes.
• [X] I have read the contributing guidelines.

Changes

• [X] Internal code
• [X] Library interface (affecting end-user code)
• [X] Documentation
• [ ] Other: _____

Closes Issue: NaN

Description

Adds ImageProxy#downloadAsIcon, same as in AttachmentProxy, this benefits users as the data is downloaded on the OkHttp threads and is then immediately usable.

Also updates some proxy docs.