Restrain window size when resizing it to 1) "keep video aspect ratio" and at the same time 2) "avoid black borders"

This is discussed in the forums here, but I can't find a matching ticket.

There are of course several choices to be made in how to implement this. I personally like it that this feature is only activated if you press and hold shift while resizing the window. This is more or less standard in image editors.

Google showed me that others have discussed this topic over at ubuntu brainstorm.