Improve display of ad breaks

[bastimeyer]

Checklist

• I understand that Streamlink Twitch GUI is just a launcher for Streamlink
• I have read the contribution guidelines
• I have checked the list of open and recently closed issues
• I have checked the commit log of the master branch

Description

Opening this as a reminder for myself to improve the display of the stream status during an ad break.

Streamlink prints messages to its log output whenever an ad break occurs, for both pre-roll and mid-roll ads. Streamlink Twitch GUI should show this status in the title of the stream popup or on the watching page whenever ads are being filtered out.

Previously, Streamlink Twitch GUI also delayed the "watching" status until pre-roll ads were finished, but something must have broken recently, and now it's showing the "watching" status even though pre-roll ads are still running.

I should probably also move the ads filtering option a little bit, as it's kinda hidden in the settings menu.

[bastimeyer]

I should probably lock this thread, and reference the stickied meta thread.

Please read this and all the linked threads / docs:
#754