Fix using icon coordinates before being allocated

When a new file is put in the desktop, it doesn't have saved coordinates, thus when adding this into a grid, it takes the coordinates of its icon as the new saved coordinates. Unfortunately, when the code gets the icon coordinates, it still hasn't been allocated by Clutter, so they are invalid.

To fix this, this patch uses the placeholder coordinates. These are valid because the extension waits until the DesktopGrid object and all its internal widgets have been allocated before adding inside the fileItems and their icons.