fix: clicking 'Cancel' on new project page does nothing

[Comnir]

☕️ Reasoning

• Clicking 'Add new project' immediately adds the new repo to the projects list
• Clicking 'Cancel' on the project setup page redirects to the app root page
• If the projects list contains a project, the root page redirects to the project page. When the project setup is incomplete, the project setup is displayed

🧢 Changes

• To avoid this redirect, clicking cancel now deletes the project
• To keep the BackButton generic, added a callback (currently named beforeOnMouseDown) to be executed before the generic implementation of back.

🎫 Affected issues

Fixes: https://github.com/gitbutlerapp/gitbutler/issues/2539

📌 Todos

• Find a better name for beforeOnMouseDown or replace with a different way to execute the deletion
• Is this the correct way to delete a project? In some place reload is called only if deleteProject didn't throw, but in other places it's called anyway.

[mtsgrd]

Thanks, this looks good. Would you mind fixing those lint errors and I'll merge this pr?

[Comnir]

Sure, sorry!

[Comnir]

@mtsgrd is there any other action required from side before you merge?

[mtsgrd]

Sorry for the delay, no not at all. Let me rebase it and merge!