[Manifest Editor] Screenshot Generation

[Jaylyn-Barbee]

• [ ] P1 - We should have a way to remove a screenshot
• [x] P1 - url to generate screenshots - why not use the existing url as a starting point instead of asking me to type it in again?
• [ ] P1 - Just pasting in a url doesn't enable the "Add URL" button - I have to hit Enter
• [ ] P0 - I noticed my screenshots were generated in dark theme - there is no way to control that - we should add an option
• [ ] P1 - Once screenshots are generated, they should be added in the list above and the fields for generation cleared
• [ ] #4546