change: Mentions: Mark messages with an @-sign and an indicator

This MR changes the way user mentions are indicated.

Coloring the whole message always comes with contrast issues, hence the change to an "@" sign indicator with a tooltip ("You have been mentioned") plus a new mention indicator bar to the left of the message. Both elements don't overlay text and come with theme-dependent contrast (Adw color variable).

Additionally, an "@" sign in the chat list indicates a mention in (one of) the unread messages.

Fixes #12489 (closed) and fixes #11251 (closed)

Chat List

Chat