Add functionality to set logo in the appearance editor

[NicEastvillage]

Show a preview of the bot's current logo and an edit button next to it. When the edit button is clicked, just open a file chooser and copy the selected png to the bot's folder and name it logo.png. There should be a some text saying 400x300 is the recommended size.

[Enderjed]

This would be incredibly useful, as I have made logos for Both Gibbus and merasmus