app: add ability to choose new layers' position.

This is a MR for future merge (after GIMP 3), and possibly improvements. We just already use it internally because the hardcoded position for new layers (always above a non-group layer or always inside — at the top — group layers) was very annoying to us, especially when @aryeom creates a lot of layers in a row (always having to move them afterwards is annoying). This is especially true for new layers when layer groups are selected.

Anyway it is here for keepsake. To be reviewed later, when we'll have time.