Ask for saving when exiting edited file

[summer-son]

Checklist

• I made sure that there are no existing issues - open or closed - to which I could contribute my information.
• I made sure that there are no existing discussions - open or closed - to which I could contribute my information.
• I have read the FAQs inside the app (Menu -> About -> FAQs) and my problem isn't listed.
• I have taken the time to fill in all the required details. I understand that the request will be dismissed otherwise.
• This issue contains only one feature request.
• I have read and understood the contribution guidelines.

Feature description

When the file was edited and not saved, currently pressing back just loses the changes without any prompt. The app should ask if changes should be saved before exiting.

Why do you want this feature?

Not to lose any changes

Additional information

Related to #107 (would be great if also implemented) but at least this feature should be implemented.