msglist: Message bounds hard to see in a run of messages from the same user

[chrisbobbe]

When we show a run of consecutive messages from the same user, it can be hard to see where one message ends and the next begins. Compare zulip-flutter and web:

web	zulip-flutter

I noticed this at #446 (review) and it came up again in beta feedback: https://chat.zulip.org/#narrow/channel/48-mobile/topic/mobile.20beta.20feedback/near/2000252

One clear improvement, I think, is to tint the background of a message when you long-press it, which you might do to open the message action sheet. We even have a Figma frame we can follow:

Once message is pressed there should be a tint of the background, since the background color could be anything. So pressedTint is a transparent foreground color.

Pressed state here is kind of selected state, because we want to keep highlighting the message as a contextual information for a dialog of editing the message.

That design has a different motivation (#126 edit message) but would help this message-bounds issue too, without having to add or rearrange anything in the layout. (Space is tight on mobile.)

EDIT: A new idea from PR #1152 that seems reasonable to me: also keep the tint while the message action sheet is open.

[Gaurav-Kushwaha-1225]

Hi @chrisbobbe,
I would like to work on this issue and implement the proposed solution.
Could you please assign it to me?
Thank you! 😊

[chrisbobbe]

Please take a look at the Zulip project's guide to getting involved with the code:

https://zulip.readthedocs.io/en/latest/contributing/contributing.html#your-first-codebase-contribution
https://zulip.readthedocs.io/en/latest/contributing/contributing.html#picking-an-issue-to-work-on