Stop icon to match play and pause icons

[FionaHuangQC]

Icon name

stop

Use cases

• in any situation involving audio, it's typical to be able to play, pause, and stop. There are currently play and pause icons, but only circle-stop and monitor-stop.
• to interrupt an ongoing process, a stop icon would be ideal

Design ideas

Checklist

• I have searched if someone has submitted a similar issue before and there weren't any. (Please make sure to also search closed issues, as this issue might already have been resolved.)
• I have searched existing icons to make sure it does not already exist and I didn't find any.
• I am not requesting a brand logo and the art is not protected by copyright.
• I am not requesting an icon that includes religious, political imagery or hate symbols.
• I have provided appropriate use cases for the icon(s) requested.

[jguddas]

There was a lot of discussions here #1043

We also need to round the corners of all those icons to be more lucidy.