Use the Desktop as a fallback destination when no write support

[aonez]

As mentioned by @MaxPower85 on #178 (comment).

• Use the save panel instead of the fallback
• Use the fallback when still no write support
• Be able to customize the fallback destination Gracefully handle non-writeable directory by using a default fallback path #500
• Let the user know the fallback was used, probably in the notification Gracefully handle non-writeable directory by using a default fallback path #500