App icons should either be in the favourites or in the app grid, not both
The goal here is to avoid duplication of the app icons. While favourites shouldn't be duplicated, running apps necessarily need to be.
- Dragging a favourite to the app grid would remove it from the dash
- Dragging a running app to the app grid wouldn't do anything - the icon would snap back to the dash on drop
- Favouriting an app using the context menu would add it to the last position in the dash
- Un-favouriting an app using the context menu would add it to the last available position in the app grid (this could perhaps be nuanced a little - we could aim to move it to the last available position on the currently visible app page; if that's full then recurse through each subsequent page until a position is found)