Enable drag & drop onto item grid

[CodeWringer]

When dragging an item onto an opened ItemGrid, attempt to fit it into the grid.

• Make sure to track where on the grid the item was dropped and then prefer placing it there, but if that is not possible, instead place it on the nearest viable slot.
• Make sure that an item can only be added to the grid, if it can at all fit onto it. If an item can't fit, add it to the property list, instead and display a warning that the item grid is full.

[CodeWringer]

Obsolete because the item grid no longer exists