Implement empty state indicators for proofread mode

This commit implements most of the proofread-mode branch of
the flowchart for empty state in Translate Extension listings:
https://commons.wikimedia.org/wiki/File:Flowchart_for_empty_state_in_Translate_Extension_listings.pdf

Bug: 46786
Change-Id: Ifd7489d001c8a3d7c4b6da8ff605b2c394319588
Merge "Implement empty state indicators for proofread mode"